How Many? A Dictionary of Units of Measurement
© Russ Rowlett and the University of North Carolina at Chapel Hill

Table of Contents
About the Dictionary
Using the Dictionary

S

sabin
a non-metric unit of sound absorption used in acoustical engineering. One sabin is the sound absorption of one square foot of a perfectly absorbing surface--such as an open window! The sound absorption of a wall or some other surface is the area of the surface, in square feet, multiplied by a coefficient which depends on the material of the surface and also on the frequency of the sound. These coefficients are carefully measured and tabulated. The unit honors Wallace Sabine (1868-1919), a Harvard University professor who founded the systematic study of acoustics about 1895. Sabine used this unit, which he called the open window unit (owu), as early as 1911.
sack [1]
a traditional unit of volume. Sacks of different commodities are of different sizes, but a typical measure is 3 bushels (about 105.7 liters based on the U.S. bushel, or 109.1 liters based on the British Imperial bushel).
sack [2]
a traditional unit of weight, varying for different commodities shipped in sacks. In Britain, for example, the sack was a traditional measure for wool, fixed by Edward III at 364 pounds (26 stone) in 1340. In the U.S., a sack of salt is traditionally equal to 215 pounds, a sack of cotton 140 pounds, and a sack of flour 100 pounds. A sack of concrete is traditionally 94 pounds in the U.S., 87.5 pounds in Canada.
sadzhen or sagene
a traditional Russian unit of distance corresponding roughly to the English fathom and French toise. Early in the 1700s, Czar Peter the Great fixed the length of the sadzhen at exactly 7 English feet. This equals 3 arshin or 2.1336 meters. Sagene is an older transliteration of the Russian word.
Saffir-Simpson category
a ranking of the strength of a hurricane, introduced by two American meteorologists and used by the U.S. National Weather Service to rank storms in the Atlantic and northeastern Pacific Oceans. A complete description, from the U.S. Atlantic Oceanographic and Meteorological Laboratory in Miami, is provided.
S.A.G. foot
the South African geodetic foot (see below).
sailmaker ounce (smoz)
a traditional unit measuring the weight (per unit area) of sailcloth. The weight in sailmaker ounces is the weight in ordinary (avoirdupois) ounces of a piece of cloth 36 inches by 28.5 inches. Thus 1 smoz is equal to 1.263 ounces per square yard (oz/yd2) or 42.828 grams per square meter (g/m2 or gsm). However, spinnakers (the large triangular foresails of yachts) are traditionally named by the fabric weight before it is finished, so these names do not correspond exactly to the sailmaker-ounce weights. A "half-ounce" spinnaker, for example, has a weight of about 0.85 smoz or 37 gsm, and a "three-quarter-ounce" spinnaker has a weight of about 1 smoz.
salmanazar
a large wine bottle holding about 9 liters, 12 times the volume of a regular bottle.
saltspoon (ssp)
a unit of volume formerly used in U.S. food recipes. The saltspoon equals 1/4 teaspoon or about 1.2 milliliters.
sao
a traditional unit of land area in Vietnam. The sao varies somewhat from province to province. It is equal to 360 square meters (430.6 square yards) in many places, but as much as 500 square meters (598.0 square yards) in others.
saros
a unit of time used in astronomy, mostly in predicting solar and lunar eclipses. The saros is equal to 6585.32 days [1] (6585 days 7 hours 23 minutes), which is exactly 223 lunar months. (This is either 10.32 or 11.32 days more than 18 years, depending on the number of leap years during the period.) Astronomers in ancient times discovered that the saros is very nearly equal to 19 eclipse years (6585.78 days). This means that one saros after an eclipse the Sun, Moon, and Earth return almost exactly to the same position and another, very similar eclipse occurs. However, because of the 7 hours 23 minutes included, the Earth has turned about one third of a revolution and the new eclipse occurs about 116° of longitude west of the preceding one. After 3 saros, the eclipse returns nearly to its original location. Thus eclipses at a particular location tend to repeat with a period of 3 saros or 54 years and about 1 month. Thus the last total solar eclipse in North Carolina, on 1970 March 7, will repeat on 2024 April 8 and again on 2078 May 11.
saturation
a measure of the brightness of a color; see color units.
savart
a unit used in music to describe the ratio in frequency between notes. The difference between two frequencies in savarts is equal to 1000 times the logarithm of the ratio between the two frequences. Thus there are 1000·log(2) = 301.03000 savarts in an octave. (In applications, this is sometimes rounded to 300.) The savart is equal to about 3.9863 cents [3] or 3.3219 millioctaves. Two notes differ by one savart if the higher note has a frequency equal to 21/301.03 = 1.002 305 times the frequency of the lower note. The unit is named for a French physicist, Félix Savart (1791-1841); although best known for his work in electromagnetism, Savart also did pioneer research in the physics of sound.
Saybolt Universal second (SSU or SUS) and Saybolt Furol second (SSF or SFS)
units of kinematic viscosity given by readings on Saybolt viscometers. The Saybolt Universal viscometer is used for liquids having viscosities below 1000 centistokes (or 10 stokes, see entry for stokes below). The Saybolt Furol viscometer is used for more viscous road and fuel oils ("furol" is an acronym for fuel and road oils). In both cases the reading is the time, in seconds, for 60 milliliters of a sample to flow through the device. The Furol viscosity readings are roughly 1/10 the Universal readings. For liquids whose viscosity exceeds 50 centistokes at 37.8 °C (100 °F) one SSU is approximately 0.2158 centistokes or 0.2158 mm2/s. For very viscous liquids (viscosity exceeding 500 centistokes) at 50 °C (122 °F), one SSF is approximately 2.120 centistokes or 2.120 mm2/s. Exact equations were published in 1996 by the American Society for Testing and Materials (ASTM practice D2161). The Saybolt seconds are considered obsolete, but they have been used traditionally in the petroleum industry and are common in technical articles.
scale
a measure describing the resolution of a map, architectural plan, or some similar document. A map, for example, might be described as "1:250 000 scale". In general, 1:n scale means that 1 unit of distance on the map or plan represents n of the same units in fact. It doesn't matter what unit of distance is used, as long as it is the same in both cases. On a 1:250 000 scale map, 1 centimeter on the map represents 250 000 centimeters (exactly 2.5 kilometers) on the ground. On the same map, one inch on the map represents 250 000 inches (about 3.9457 miles) on the ground. The use of the terms "larger" and "smaller" referring to scale is often confusing. A 1:m scale map has larger scale than a 1:n scale map if m is less than n. For example, a 1:100 000 scale map has larger scale than a 1:250 000 scale map, and the same place or object will appear larger on the larger scale map. Mathematically, the notation 1:n is simply another way of writing the ratio 1/n.
sccm, scfm, scfh, scfd, scim (etc.)
symbols for "standard cubic centimeters per minute", "standard cubic feet per minute", "standard cubic feet per hour", "standard cubic feet per day", and "standard cubic inches per minute." Many similar abbreviations are in use. These are units of flow rate for gases, and the term "standard" indicates that the flow rate assumes a standard temperature and a standard pressure of 1 atmosphere. There is some variation in the standard temperature. For natural gas, the petroleum industry uses a standard temperature of 60 °F (15.6 °C). For air flow, the standard temperature is sometimes 32 °F (0 °C) or 68 °F (20 °C) and a standard relative humidity must also be specified. The actual air flow is often designated with an "a" instead of an "s", as in "acfm".
scheffel or schepel
traditional units of dry volume. The German scheffel and Dutch schepel have both been redefined within the metric system, but in very different ways: the scheffel equals 50 liters (1.4189 U.S. bushels) and the schepel 10 liters (0.2838 U.S. bushels). Both words are usually translated "bushel" in English, and both units were originally closer to the English bushel; the schepel was roughly 0.75 bushel or about 26 liters. See also skep (below).
schock
a traditional German unit of quantity equal to 60. See shock [1] below.
schooner
an informal unit of liquid volume. A schooner is a large tumbler or drinking glass holding about 400 milliliters or 13.5 U.S. fluid ounces. Similarly, in Queensland, New South Wales, and the Northern Territory (Australia) a schooner of beer holds 425 milliliters. In South Australia, however, a schooner is only 285 milliliters.
schoppen
a traditional German unit of liquid volume for wine, now interpreted most often as 250 milliliters (1/4 liter or about 8.45 U.S. fluid ounces).
schtoff
a traditional Russian unit of volume equal to 10 charki. This is equivalent to about 1.23 liters or 1.30 U.S. liquid quarts.
scim
see above under sccm.
score
a traditional unit of quantity equal to 20. The score, like the dozen, helps us describe a moderate number of objects. This is one of the many cases in which English has two words for similar concepts, one word from the Old French spoken in 1066 by the Norman conquerors of England and one from the Old English spoken by the Anglo-Saxon people they conquered. In this case, dozen is the French word and score is the Old English word, derived from the Norse word skor meaning a notch cut in a stick as a tally mark. The suffix -score can be added to a number, as in threescore (60) or fivescore (100).
Scots foot, Scots mile
traditional distance units in Scotland. The Scots foot equals 1.005 405 4 English foot (about 12.065 English inches or 30.645 centimeters). The Scots mile equals 320 falls or 5920 Scots feet, which is about 5952 English feet (1.127 English mile or 1814.2 meters). The English foot and statute mile, of course, are now official in Scotland.
Scoville unit
a unit measuring the concentration of capsaicin, the "hot" ingredient in chile peppers. A measurement of, say, 50 000 Scoville units means that an extract from the pepper can be diluted 50 000 to 1 with sugared water and the "burn" of the capsaicin will still be barely detectable by the human tongue. (In practice, the measurements are now made with liquid chromatography.) The unit was invented in 1912 by the American pharmacologist Wilbur L. Scoville, who was working on the use of capsaicin in the muscle pain-relieving ointment Heet. Actual chile peppers have capsaicin concentrations from 5000 to 500 000 Scoville units.
scruple (s)
a unit of weight in the traditional (troy) system used by English apothecaries, equal to 20 grains, 1/24 troy ounce [2] or approximately 1.2960 gram. See also dram [2], and see troy weights for additional information. The name of the unit is from the Latin scrupulus, meaning a small, sharp stone. (This word also came to mean "something which causes pain or annoyance," such as a pebble in one's shoe, and this led to our other use of the word "scruple" to mean an ethical consideration.) Units similar to the scruple were used throughout Europe; the French and Russian scrupule, Italian scrupolo, and German skrupul are equal to 20 of the local unit corresponding to the grain and all are equivalent to something in the range 1.1-1.3 grams.
se
a traditional Japanese unit of area equal to about 99 square meters or 118.4 square yards.
seah
an ancient Hebrew measure of both liquid and dry volume. The seah was equal to about 13.44 liters (about 3.55 U.S. liquid gallons or 2.96 British Imperial gallons).
seam
a traditional unit of volume. A seam of grain was 8 bushels: this would be equivalent to 290.95 liters based on the British Imperial bushel, or 281.91 liters based on the older U.S bushel. Take your pick. The Anglo-Saxon word "seam" meant the load of a pack animal. (The same word also came to mean the stitching of the packsaddle, and that's the origin of our other uses of the word today.) We don't know how large the seam was in Saxon times, but it was equal to 8 bushels at least by the end of the thirteenth century. The seam continued in use to the early nineteenth century, but in later years it was more often called a quarter [5]. The unit was sometimes called the soam.
sea mile
another name for the nautical mile.
season
a portion of a year. The word "season," derived from a Latin word meaning the time for sowing, originally meant one of the periods of the agricultural year. It has come to be used informally to mean the period of time characterized by any activity (such as the "football season") or more specifically as an informal unit of time equal to roughly 1/4 year. Ideas of what constitutes a season in this latter sense vary from country to country. In North America, the astronomical seasons begin at the instants of equinox (for spring and fall) or solstice (for summer and winter), on or near the 21st days of March, June, September, and December. In meteorology, however, the seasons begin on the 1st days of March, June, September, and December.
second (s or sec or ") [1]
a fundamental unit of time in all measuring systems and the SI base unit of time. The name simply means that this unit is the second division of the hour, the minute being the first (see minute [0]). The second was defined as 1/86 400 mean solar day until astronomers discovered that the mean solar day is actually not constant (see day [2]). The definition was then changed to 1/86 400 of the specific mean solar day 1900 January 1. Since we can't go back and measure that day any more, this wasn't a real solution to the problem. In 1967, scientists agreed to define the second as that period of time which makes the frequency of a certain radiation emitted by atoms of cesium-133 equal to 9 192 631 770 hertz (cycles per second). In other words, if we really want to measure a second, we count 9 192 631 770 cycles of this radiation. This definition allows scientists to reconstruct the second anywhere in the world with equal precision.
second (" or s or sec) [2]
a unit of angular measure equal to 1/60 arcminute. This unit is also called the arcsecond to distinguish it from the second of time. One second is a very small angle indeed: there are 1 296 000 seconds in a circle. The SI defines s as the symbol for the time unit (see above) and recommends " as the symbol for the arcsecond. The international standard ISO 31 recommends that angles be stated in degrees and decimal fractions of the degree, without use of arcminutes and arcseconds.
second (" or s or sec) [3]
a unit of longitude used in astronomy. Astronomers measure right ascension in time units by dividing the equator into 24 hours instead of 360 degrees. (Right ascension is the longtitude coordinate for positions in the sky; see hour [2]). This makes 1 second of longitude equal to 15 arcseconds.
second (" or sec) [4]
a unit of viscosity defined by the time required for a specified amount of a liquid to flow through a particular viscometer. The Saybolt second (see above) was used in the U.S., the Redwood second in Britain, and the Engler degree in continental Europe.
second (s or sec) [5]
a unit measuring the fuel efficiency of rocket engines. The measure is obtained as the thrust of the engine divided by the weight of fuel consumed per second (weight being measured at the earth's surface, or at 1 g). Since both thrust and weight are forces, their units cancel and the ratio is measured in seconds. This ratio is often called "specific impulse," although the true specific impulse (measured in speed units) is the ratio of the thrust to the mass (not weight) of fuel consumed per second.
second (s or ") [6]
a unit of sidereal time in astronomy; see "sidereal day" below.
second-day-foot (sdf)
a unit of volume for water sometimes used in U.S. hydrology. A second-day-foot is the volume of water accumulated in one day by a flow of one cubic foot per second; this is equal to exactly 86 400 cubic feet or about 2446.58 cubic meters. One second-day-foot is equivalent to approximately 1.9835 acre feet. Also known as the day-second-foot (dsf).
second-foot
an informal name for the cubic foot per second as a flow rate for water.
section (sec)
a traditional unit of area in the U.S. and western Canada, nominally equal to 1 square mile. This unit is used by the U.S. Public Land Survey System, which applies to most of the U.S. except for the original 13 states, Alaska, and Hawaii, and by the similar Dominion Lands Survey System in the four western provinces of Canada. Because sections were surveyed using grids of north-south and east-west boundaries, their actual size varies slightly (by less than 1%) due to the Earth's curvature, depending on their position within the specific grid. Nominal quarter-section lots in the U.S. would also be slightly smaller because space was taken from them for roads, but Canada allowed space for roads between adjacent sections.
SED
a symbol for standard erythemal dose, a unit used to measure the amount of skin-reddening ultraviolet radiation received by a person in the sun or in a tanning salon. One SED is equal to a dose of 100 joules per square meter (J/m2) of skin surface. The radiation producing skin reddening is confined to a narrow range of wavelengths around 300 nanometers, so the energy is measured only in these wavelengths using a standardized procedure. A tanning rate of one SED per hour is equivalent to 27.778 milliwatts per square meter (mW/m2) of skin surface.
seemeile
the German name for the nautical mile.
seer [1]
a traditional weight unit in India and South Asia. The seer equals 1/40 maund, and, like the maund, it varied considerably from one area to another. The official size in British India was 2.057 15 pounds or 0.9331 kilogram. In Pakistan, the seer is now considered equal to the kilogram. The unit is sometimes spelled ser.
seer [2]
a traditional unit of dry volume in northern India, equal to a little more than a liter. This is roughly the volume of a seer [1] of grain.
SEER
an abbreviation for seasonal energy efficiency rating, a U.S. and Canadian measure of the efficiency of an air conditioner. The rating is equal to the total output of the air conditioner over an entire cooling season, in Btu, divided by the total electrical energy consumed, in watt hours. Since this is a ratio of two energy units, the result is a dimensionless (unitless) number. In 2005 the required SEER for home central air conditioning systems was raised from 10 to 13. See also EER and COP.
Seidel
a traditional unit of liquid volume in Austria. The traditional Seidel was equal to about 354 milliliters; this is about 12.0 U.S. fluid ounces or about 12.5 British fluid ounces. In southern Germany the Seidel was larger, about 535 milliliters or 18.09 U.S. fluid ounces. Today a Seidel of beer in southern Germany and Austria is a small mug holding 300 milliliters (10.14 U.S. fluid ounces) in Austria or 500 milliliters (16.91 U.S. fluid ounces) in Germany.
semester (sem)
an informal unit of time. The word semester comes from the Latin words for "six months," and originally a semester was understood to equal 6 months or 1/2 year. However, the word is now used chiefly to mean half the academic year at a school or college, a period of time which can vary from 15 to 21 weeks.
semester hour (sem hr)
a unit of academic credit, supposedly equal to one semester's study for a period of one hour per week. However, "academic hours" slightly shorter than regular hours (often 50 or 55 minutes per class) are typically used in these calculations.
semi-
a common English prefix meaning 1/2. In statements of frequency, bi- and semi- have become confused and it isn't always clear what a word like "semimonthly" means. This is how it's supposed to work: in adverbs of frequency, semi- means "twice every" or "every half." Bells on a ship ring semihourly (every half hour) and the tides usually occur semidiurnally (twice a day); a semiweekly newspaper is published twice in a week; a semimonthly payroll is paid twice every month; and days and nights have the same length semiannually (twice a year). For something that happens once every two time units, use bi-.
semibreve
a unit of relative time in music equal to 1 whole note or 1/2 breve.
semicircle
a unit of angle measurement equal to 1/2 circle, pi radians, or 180°.
semih
abbreviation for semihourly (every half hour) sometimes used in medical prescriptions.
semiquaver
a unit of relative time in music equal to 1/16 whole note or 1/32 breve.
semitone
a unit used in music to describe the ratio in frequency between notes. The unit is actually used in two slightly different ways. In one use, two notes are said to differ by one semitone if the higher note has frequency exactly 16/15 = 1.0667 times the frequency of the lower one. Also, the semitone is used as a synonym for the half step in the standard chromatic scale; in this use, two notes differ by a semitone if the higher note has frequency exactly 21/12 = 1.0595 times the frequency of the lower one.
sennight
an old English name for a week, formed as a contraction of seven nights. The word is pronounced like "senate."
septennium
a unit of time equal to 7 years.
septet
a unit of quantity equal to 7.
septuple, septuplet
a group of 7 items, especially 7 identical items; the word septuplet is also used for one member of the group.
ser
see seer (above).
sester
an old English unit of measure for honey. It seems to have been equal to about 2 U.S. gallons, 1.7 Imperial gallons, or 7.5 liters.
sestet
another name for a sextet, a unit of quantity equal to 6. This spelling is used in poetry to describe a six-line stanza and is sometimes used in music for an ensemble of 6 instruments.
seven
a unit of volume for beer in New South Wales and some other sections of Australia. A seven of beer is a glass holding 200 milliliters (about 7 Imperial fluid ounces). This volume is called a butcher in South Australia and a glass [3] many other parts of Australia.
seventh
a unit used in music to describe the ratio in frequency between notes. Two notes differ by one seventh if the higher note has frequency exactly 15/8 times the frequency of the lower one. On the standard 12-tone scale, the seventh is approximated as 11 half steps, corresponding to a frequency ratio of 211/12 = 1.8877.
sextarius
a Roman unit of liquid volume. The word means "sixth", and the unit was equal to 1/6 congius. The sextarius held about 530 milliliters, roughly the capacity of the British and U.S. pints (568.261 and 473.176 milliliters, respectively).
sextet
a unit of quantity equal to 6.
sextuple, sextuplet
a group of 6 items, especially 6 identical items; the word sextuplet is also used for one member of the group.
SF
a common symbol for the square foot (ft2).
SFM
a traditional symbol for surface feet per minute, a unit measuring the rate at which a rotating tool, such as a lathe or saw, processes material. The term surface foot refers to the length of the material, similar to the linear foot.
SG
a common symbol for specific gravity, sometimes appended to a measurement as if it were a unit, as in "1.134 SG" for a specific gravity of 1.134. (Specific gravity is a mathematical ratio of densities, so it has no unit.)
shackle
a traditional unit of length used for measuring the lengths of nautical cables and chains, especially anchor chains. Anchor chains are formed by using shackles to join short lengths of chain. When the anchor is dropped and the chain runs out, a seaman counting the number of shackles can report the total length of chain deployed. The size of the unit therefore varied somewhat, depending on the length of the short chains used. In Britain, the unit became standardized in the 16th century at 12.5 fathoms (75 feet or 22.86 meters). In 1949, the Royal Navy adopted a length of 15 fathoms, which is 90 feet or 27.432 meters; this brought the British shackle in line with the U.S. unit, which is usually called the shot.
shade number
a unit of light transmission for the protective glasses used in welding. If T is the fraction of visible light transmitted, the shade number is 1 + 7(-log10 T)/3. For example, if 1% of the light is transmitted, the shade number is 4.
shaftment
an old English unit of distance equal to 2 palms. A shaftment is the distance from the tip of the outstretched thumb to the opposite side of the palm of the hand. The ending "-ment" is from the old English word mund, hand. The shaftment was an important unit in Saxon England, where it was equal to about 16.5 centimeters (6.5 inches). After the modern foot came into use in the twelfth century, the shaftment was reinterpreted as exactly 1/2 foot or 6 inches (15.24 centimeters). The shaftment continued in common use through at least the fifteenth century, but it is now obsolete.
shake
an informal unit of time equal to 10-8 second or 10 nanoseconds (ns). This unit originated in nuclear physics. In an atomic explosion, fast-moving neutrons break apart atoms of uranium or plutonium; the fission of these atoms releases additional neutrons which keep the reaction going. The shake is the approximate lifetime of an individual neutron. The word shake and the expression "shake of a lamb's tail" have long been used in English to mean a very brief period of time.
shaku
a Japanese word meaning "measure" or "scale", also used for several traditional units in Japan: [1] As a unit of distance, the shaku is the Japanese foot, equal to about 30.30 centimeters or 11.93 inches; [2] As a unit of area, the shaku equals 330.6 square centimeters (51.24 square inches); [3] As a unit of volume, the shaku equals about 18.04 milliliters (0.61 U.S. fluid ounce).
shannon (Sh)
a unit of information content used in information and communications theory. The definition is based on the idea that less-likely messages are more informative than more-likely ones (for example, if a volcano rarely erupts, then a message that it is erupting is more informative than a message it is not erupting). If a message has probability p of being received, then its information content is -log2 p shannons. For example, if the message consists of 10 letters, and all strings of 10 letters are equally likely, then the probability of a particular message is 1/2610 and the information content of the message is 10(log2 26) = 47.004 shannons. This unit was originally called the bit [2], because when the message is a bit string and all strings are equally likely, then the information content turns out to equal the number of bits. One shannon equals log10 2 = 0.301 030 hartley or loge 2 = 0.693 147 nat. The unit is named for the American mathematician Claude Shannon (1916-2001), the founder of information theory.
sheaf
a traditional measure for grain and various other commodities. A sheaf of grain is a bundle of stalks having a standard size, typically 30 to 36 inches (roughly 75-90 centimeters) in circumference. A sheaf of arrows is a bundle or quiver of 24.
shed
an exceptionally small unit of area used in particle physics. Like its big brother, the barn, the shed is used to express the apparent cross-sectional area of a particle from which other particles are scattered. One shed equals 10-24 barn, which is 10-52 square meter or 0.0001 square yoctometer (ym2).
shekel or sheqel
an ancient Hebrew unit of weight (and also a coin having that weight). The shekel was the Hebrew version of a Babylonian unit used throughout the Middle East. Accounts differ on its size. A frequently quoted equivalent is 1/60 mina, which is equal to about 8.3 grams or 0.29 ounce (avoirdupois); other sources quote a value of 11.3 grams or 0.40 ounce or various other values in the range 7-14 grams (0.25-0.5 ounce).
sheng
a traditional unit of liquid volume in China. Like the Indian seer (see above), the sheng is a little more than a liter; 1.035 liter (1.094 U.S. quart) is one quoted equivalent.
shetland
a unit of volume for beer in Western Australia, equal to 115 milliliters (4 Imperial fluid ounces). This quantity is a smaller version of the 5-ounce pony; its name refers to Shetland ponies, small horses from the Shetland Islands north of Scotland.
shift
a unit of time equal to the scheduled period of work at a factory or other place of business. Businesses operating on a 24-hour basis typically organize the day into three daily shifts of 8 hours each. This usage is consistent with the old English meaning of "shift" as an arrangement or division.
shipping ton
a traditional unit measuring the volume (not the weight) of items shipped by sea. The U.S. shipping ton is the same as the freight ton (see ton [5]), 40 cubic feet or about 1.1326 cubic meters. The British shipping ton is 5% larger at 42 cubic feet (1.1893 cubic meters).
sho
a traditional Japanese unit of liquid volume. The sho equals 1.8039 liter, which is 1.9061 U.S. quarts or 1.5872 British Imperial quarts.
shock or shook [1]
a unit of quantity equal to 5 dozen, 3 score, or 60. The unit is more common in German, where the word is spelled schock, than it is in English. In cooperage (the making of barrels), a bundle of 60 barrel staves is traditionally called a shock.
shock or shook [2]
a traditional measure of grain or straw. A shock of grain is usually 12 sheaves (see above), sometimes 10.
shoe size
All shoe sizes express in some way the approximate length of the shoe, or at least the length of the last, the form on which the shoe is made. In the U.S., a difference of one full shoe size represents a length difference of 1/3 inch (8.47 mm), so shoe size n represents a length of Z + n/3, where Z is the length of a size 0 shoe (if there were such a thing). The value of Z is 3-11/12 inches (99.5 mm) for infants' and boys' shoes, 3-7/12 inches (91.0 mm) for girls' shoes, 7-11/12 inches (201.1 mm) for women's shoes, and 8-1/4 inches (209.6 mm) for men's shoes. The size number for a woman's shoe is 1 larger than for a man's shoe of the same length (for example, a man's 7-1/2 is the same length as a woman's 8-1/2). In Europe, shoe sizes are measured in Paris points, a unit equal to 2/3 centimeter. Ski boots and hiking boots worldwide are measured in mondo points, which are simply millimeters. Link: Wikipedia has information on other shoe size systems used in Britain, Japan, and Australia.
Shore hardness
a hardness measure determined by a Shore durometer. See duro.
short hundredweight
a common name for the U.S hundredweight or cental (100 pounds).
short ream
the traditional ream of 480 sheets of paper. The standard size for the ream has been increased in recent years to 500 sheets.
short ton (st or tn)
a common name for the U.S. ton (2000 pounds).
short ton unit (stu)
a unit of mass used in mining to measure the mass of the valuable metal in an ore. Customarily, the short ton unit is defined to be one U.S. ton of ore containing 1% metal, but it is the metal, not the ore, that is being measured. Thus the unit is really a unit of mass equal to 20 pounds (about 9.072 kilograms).
shortword
a unit of information generally equal to 1/2 word [2].
shot [1]
a traditional unit of liquid volume. The term "shot" is often used informally to mean "a small serving." In the U.S. a shot is usually the same as a jigger: 1.5 fluid ounces or 44.4 milliliters. However, some bartenders use shot glasses of other sizes such as 1.25 fluid ounces (37.0 milliliters).
shot [2]
another name for a shackle as unit of length for anchor chains. In this use, common in the U.S., a shot equals 15 fathoms, 90 feet or 27.432 meters. The origin of this name for a length of chain is not certain. It may be a corruption of "shut," since anchor chains are formed by shutting clasps or shackles to join shorter lengths of chain.
shovel
an informal unit of volume. In U.S. building trades, a common rule of thumb is that a cubic yard contains about 150 standard (no. 2) shovels of material. This means that a shovel contains about 5 liters and a cubic meter is about 200 shovels.
shower unit
a unit of distance used in studying cosmic radiation; it is the distance a charged particle travels in a given material while its energy decreases by 50%. Thus it depends on the material: in air the shower unit is very long, about 230 meters, but in water it is only 30 centimeters.
sidereal day
a unit of time used in astronomy, equal to the period of time in which the earth makes one rotation relative to the stars. If we could view the earth from outside the Solar System, we would see that it actually completes 366.242 rotations during one year (one revolution around the sun). We only count 365.242 because one rotation is cancelled out for us by our tour around the sun. Thus the sidereal day, the average interval between two successive risings of the same star, is shorter than the mean solar day (see day) by 1/366.242. The sidereal day equals 23 hours 56 minutes 4.090 54 seconds, or 86 164.090 54 seconds. Like the regular day it is divided into 24 sidereal hours, each sidereal hour being divided into 60 sidereal minutes and each sidereal minute into 60 sidereal seconds. The sidereal hour equals 59 minutes 50.17 seconds; the sidereal minute equals 59.8362 seconds, and the sidereal second equals 0.997 270 second. Traditionally, observatories had clocks set to this sidereal cycle, and astronomers still use sidereal time in making telescope settings.
Siegbahn unit
another name for the X unit, a unit of distance used for describing the wavelength of x-rays. The name honors the Swedish physicist Karl M.G. Siegbahn (1886-1978), who received the Nobel Prize in Physics in 1924 for his work in x-ray spectroscopy.
siemens (S)
the SI unit of electric conductance, susceptance, and admittance. The most important property of a conductor is the amount of current it will carry when a voltage is applied. Current flow is opposed by resistance in all circuits, and by also by reactance and impedance in alternating current circuits (see ohm). Conductance, susceptance, and admittance are the inverses of resistance, reactance, and impedance, respectively. To measure these properties, the siemens is the reciprocal of the ohm. In other words, the conductance, susceptance, or admittance, in siemens, is simply 1 divided by the resistance, reactance or impedance, respectively, in ohms. The unit is named for the German electrical engineer Werner von Siemens (1816-1892). See also mho.
sieve [1]
a traditional measure of the fineness of a wire screen (such as the screening used in a sieve). Higher sieve numbers correspond to finer screens. In The U.S., 10 sieve fabric has openings of 2 millimeters; 100 sieve has openings of 0.15 millimeters. A similar but slightly different scale was used in Britain. In the metric system, sieve fineness is specified by the diameter of the openings, in millimeters or micrometers. Link: sieve comparison table from Screen Technology Group, Inc.
sieve [2]
a flat basket for fruits and vegetables, sometimes used as unit of volume in southeastern England. A sieve is approximately equal to a bushel [1], that is, about 35 liters.
sievert (Sv)
an SI unit used for measuring the effective (or "equivalent") dose of radiation received by a human or some other living organism. Various kinds of radiation have different effects on living tissue, so a simple measurement of dose as energy received, stated in grays or rads, does not give a clear indication of the probable biological effects of the radiation. The equivalent dose, in sieverts, is equal to the actual dose, in grays, multiplied by a "quality factor" which is larger for more dangerous forms of radiation. An effective dose of one sievert requires 1 gray of beta or gamma radiation but only 0.05 gray of alpha radiation or 0.1 gray of neutron radiation. One sievert equals 100 rem. The sievert is a large unit, so radiation doses are often measured in millisieverts (mSv). The unit honors the Swedish physicist Rolf Sievert (1898-1966), who worked over many years to measure and standardize the radiation doses used in cancer treatment.
sigma
an informal name for the standard deviation (see below). The lower case Greek letter sigma is the usual symbol for the standard deviation.
sign
an informal unit of angle measure originating in astrology. The Sun's annual path through the sky, called the Zodiac, is divided into 12 parts called signs, each sign corresponding roughly to one of the classic twelve constellations through which the Sun passes. Thus 1 sign is equal to 1/12 circle, or 30°.
sitio
a name used traditionally in Mexico and the southwestern U.S. for the square legua (see legua [2]), a unit of area equal to 25 million square varas or about 1800 hectares. The word means "place."
sixth
a unit used in music to describe the ratio in frequency between notes. Two notes differ by one minor sixth if the higher note has frequency exactly 8/5 times the frequency of the lower one, or by a major sixth if the higher note has frequency exactly 5/3 times the frequency of the lower one. On the standard 12-tone scale the minor sixth is approximated (roughly) by 8 half steps, corresponding to a frequency ratio of 22/3 = 1.5874; the major sixth is approximated by 9 half steps, corresponding to a frequency ratio of 23/4 = 1.6818.
skein
a coil of yarn or cord. In retail trade, a skein is a highly variable unit, varying from one type of yarn to another and often from one manufacturer to another. In textile manufacturing, however, the skein is a unit of length equal to the lea.
skep, skæppe, skjeppe
a traditional unit of dry volume in northern Europe. Use of the unit was spread by Norse traders. In Britain, the skep was eventually identified with the bushel and the word survives as a name for a farm basket or a domed beehive. The Danish skæppe and Norwegian skjeppe are equal to 18 pots; this is equivalent to 17.407 liters (0.494 U.S. bushel) in Denmark and 17.370 liters (0.493 U.S. bushel) in Norway. The Dutch schepel and German scheffel (see above) are versions of this same unit.
skock
a traditional Swedish unit of quantity equal to 60. See shock [1] above.
skot
a unit of (low) luminance equal to 0.001 apostilb. This unit was used in Germany during World War II to describe permitted levels of lighting during air raids. One skot is the brightness of a surface produced by 1 nox or 0.001 lux of light. The name comes from the ancient Greek skotos, darkness.
slinch
a unit of mass invented by the U.S. National Aeronautics and Space Administration (NASA). The unit is part of a system based on the pound of force and the inch. One slinch is the mass accelerated at one inch per second per second by a force of one pound; thus the slinch equals exactly 12 slugs (see below) or about 386.088 pounds (175.1268 kilograms). The word is a contraction of slug-inch. In the U.S. military aircraft industry, this unit is sometimes called a mug. It has also been called a snail.
slm, slpm
symbols for standard liters per minute. 1 slm = 1000 sccm; see above under sccm.
slug
a unit of mass in the English foot-pound-second system. One slug is the mass accelerated at 1 foot per second per second by a force of 1 pound. Since the acceleration of gravity (g) in English units is 32.174 04 feet per second per second, the slug is equal to 32.174 04 pounds (14.593 90 kilograms). The slug was formerly used in calculations in mechanics and engineering, but it has been largely abandoned in favor of metric units. The unit was called the "engineer's mass unit" during the late nineteenth century. The British physicist A. M. Worthington first called it a slug in a 1902 textbook. (Probably he had in mind older uses of the word to mean a weight or a projectile. In the 1600's a slug was a roughly shaped lump of metal shot from a primitive cannon.)
slyke
a unit of buffer value in chemistry, defined to be the concentration of acid or base that must be added to a solution to change its pH by one unit. One slyke is equal to one millimole of acid or base per liter of solution (mmol/L). The unit is attributed to and named for D.D. Van Slyke (1883-1971), an American physician and chemist.
sm
a symbol sometimes used in English for the statute mile (5280 feet or 1609.344 meters).
smidgen
a very small quantity of material. Until recently, no one thought a smidgen was an actual unit of measure, but recently kitchen supply stores in the U.S. and other countries have begun selling sets of "minispoons" in which the smallest spoon, labeled "smidgen," is designed to hold exactly 1/2 pinch or 1/32 teaspoon, which is roughly 0.005 fluid ounce or 0.15 milliliter. The word is a diminutive of "smutch" or "smudge"; it originally meant a small spot.
smite
an old English word for a small amount of something; in recipes, a pinch. Originally the word probably meant one of the pieces into which something had been smashed.
smoot
a humorous unit of distance invented in 1958 by a fraternity at the Massachusetts Institute of Technology. The fraternity pledges of Lambda Chi Alpha measured the length of Harvard Bridge using pledge Oliver R. Smoot ('62). According to Smoot himself, the bridge turned out to be 364.4 smoots long "plus epsilon," but this has been recorded as 364.4 smoots "plus an ear." The bridge is still marked in smoots. Proposals to change the definition of the unit by remeasuring it with Smoot's son Steve (MIT '89) or daughter Sherry ('99) were rebuffed. One smoot equals 67 inches (170.18 centimeters). Oliver Smoot became an attorney but continued his interest in standards and measurement. He is a past Chairman of the Board of Directors of the American National Standards Institute (ANSI), and he was President of the International Organization for Standardization (ISO) in 2003-04.
smoz
symbol for the sailmaker ounce (see above).
snail
another name for a slinch (see above).
Snellen fraction
a ratio (such as 20/20 or 20/100) measuring the acuity (sharpness) of a person's eyesight for objects at a distance. The denominator (number after the slash) is the distance at which the detail of a letter on a standard test chart would subtend an angle of one arcminute, and the numerator (number before the slash) is the testing distance, the distance at which the person correctly identifies the letter. By custom, the ratio is often stated with a standard testing distance such as 20 feet or 6 meters; thus 20/100 (or 6/30) means that the person located 20 feet (or 6 meters, respectively) from the chart can identify a letter subtending an angle of one arcminute at a distance of 100 feet (30 meters). The fraction is also stated as a percentage, for example "20% vision" rather than 20/100. The unit is named for the Dutch ophthalmologist Herman Snellen (1834-1908), who developed the first standard methods for measuring visual acuity.
snit
a U.S. unit of volume for liquor equal to 2 jiggers, 3 U.S. fluid ounces, or 88.7 milliliters. The origin of this unit is unknown.
soam
another name for a seam (see above).
soendre
a traditional unit of land area in Bhutan, equal to roughly 1/20 acre or about 200 square meters.
sol
a unit of time on Mars, equal to the average length of the Martian day as it would appear to an observer on the red planet. The sol equals 24 hours 39 minutes 35 seconds, which is 88 775 seconds or about 1.0275 Earth days.
solar neutrino unit (snu)
a unit used by astrophysicists to measure the rate at which neutrinos from the Sun are detected on Earth. Neutrinos are elementary particles with no charge and little or no mass (in 1999 it was reported that neutrinos do have a small mass). Scientists want to detect neutrinos because they carry crucial information about processes deep inside the Sun, but neutrinos are very hard to catch: most of them pass right through the Earth without interacting with anything. The solar neutrino unit, therefore, is extremely small: it is defined to be 10-36 neutrino capture per target atom per second.
-some
a suffix added to a number to create a unit of quantity. For example, a foursome is a group of four. The suffix comes from the use of "some" as a pronoun meaning a number of individuals, as in "Some Like It Hot."
sone
a unit of subjective sound loudness. One sone is the loudness of a pure tone of frequency one kilohertz and strength 40 decibels. A sound has loudness s sones if a listener judges it to be s times louder than a sound of 1 sone. The phon is another unit of sound loudness; a sound of loudness p phons has loudness 2(p - 40)/10 sones. The sone is often used in industrial engineering to express the perceived loudness of engines, fans, and other items of industrial equipment. The unit was introduced by S.S. Stevens and H. Davis in 1938. Its name is from the Latin word sonus, sound.
sotka
a Russian name for the are, a metric unit of area equal to 100 square meters. This unit is commonly used to state the areas of small tracts of land. One sotka is approximately 1076.4 square feet, 119.60 square yards, or 0.02471 acre. The unit is sometimes transliterated from the Cyrillic as cotka.
sound transmission class (STC)
a rating of the ability of building materials to stop the transmission of unwanted sounds. The rating is designed so that it expresses, as best one can with a single figure, the sound transmission loss in decibels when sound passes through the material. To determine the STC rating, test sounds of 16 different frequencies are transmitted through the barrier. The data is then smoothed to a standard contour using an established protocol, and the STC rating is the "smoothed" sound transmisson loss at a frequency of 500 hertz. Ordinary conversation can be understood through a window pane rated STC 25; a wall rated STC 50 stops all but the loudest shouting.
South African geodetic foot (S.A.G. ft)
a unit of distance used for surveying and geodetic measurement throughout southern Africa. The S.A.G. foot is slightly shorter than the standard British or American foot; it equals 30.479 726 54 centimeters or 0.999 991 898 49 foot.
span
a traditional unit of distance equal to 9 inches (approximately 22.9 centimeters) or 1/4 yard. This distance represents the span of a man's hand with fingers stretched out as far as possible. The Old English word spann meant precisely this unit of measure; all the other uses of the word "span" came later.
spat [1]
a unit of solid angle measure equal to the sphere, that is, 4(pi) steradians (see below). This usage comes from the Latin word spatium, space, since a solid angle of 1 spat covers all of space surrounding the vertex of the angle.
spat [2]
an informal unit of distance formerly used by astronomers. The spat is equal to the terameter (Tm, or 1012 meters). This is equivalent to about 6.6846 astronomical units. The origin of this usage is not clear; "spat" may be an acronym for "space unit" or "space-time".
SPF
abbreviation for "sun protection factor," a rating for lotions protecting against sunburn. The rating indicates how many times longer a person who uses the lotion can remain in the sun with an equivalent effect. For example, a person who would not sunburn in 10 minutes without protection can remain in the sun up to 150 minutes by using a lotion rated SPF 15.
sphere
the traditional unit of solid angle measure, divided into 4(pi) steradians (see below). There are also 129 600/(pi) = 41 252.96 square degrees in a sphere. This unit is also called the spat (see above).
spherical degree
a unit of relative surface area for spheres, equal to 1/720 the total surface area or pi·R2/180, where R is the radius of the sphere. Thinking in terms of the Earth's surface, this is the area of the region in one hemisphere (northern or southern) bounded by the equator and two meridians of longitude one degree apart.
spin
a unit of angular momentum used in particle physics. The spin unit is equal to Planck's constant h divided by 2pi, or approximately 105.457 27 x 10-36 joule second (J·s). The spin of an elementary particle is always a simple multiple of this unit.
spindle
a traditional measure of length used for yarn. The length varied with the material; a spindle of cotton yarn, for example, was 15 120 yards (13.826 km), and a spindle of jute was 14 400 yards (13.167 km). The cotton spindle was also equal to 18 hanks.
splash
see dash [2].
split
a unit of volume for liquor equal to 4 jiggers, 6 U.S. fluid ounces, or 177.4 milliliters. The same name is used for a small wine bottle holding 1/4 the volume of a regular bottle; this would be about 187.5 milliliters. The unit appears to have originated as half the volume of a 12-ounce bottle of soda, which bartenders would split between two drinks.
spoonful
an informal unit of volume, sometimes an alternate name for the teaspoon.
square (sq) [1]
a traditional unit of area, often used for measuring roofing material, finished lumber, and other building materials. One square equals 100 square feet, 11.111 square yards, or about 9.290 square meters.
square [2]
relative to resistivity measurements, see ohm per square.
square block
see block [3].
square centimeter (cm2)
the CGS unit of area. 1 cm2 equals 100 mm2, 10-4 m2, or approximately 0.155 000 3 in2.
square chain (ch2 or sq ch)
a traditional unit of area in English surveying, defined to be the area of a square whose side equals Gunter's chain (4 rods [1] or 22 yards). Thus the square chain equals 16 square rods, 484 square yards, 4356 square feet, or exactly 0.1 acre. In metric terms, the square chain equals about 4.0569 ares or 405.69 square meters.
square degree (sq deg)
a unit of solid angle measure. Since one degree of ordinary angle measure equals pi/180 radians, one square degree is defined to be (pi/180)2 = 0.000 304 617 4 steradian. There are 4·1802/(pi) = 129 600/(pi) = 41 252.961 25 square degrees in a sphere.
square foot (ft2 or sq ft)
a traditional unit of area. 1 ft2 equals 144 in2 or exactly 929.0304 cm2 (0.092 903 04 m2).
square inch (in2 or sq in)
a traditional unit of area. 1 in2 equals exactly 6.4516 cm2 or 645.16 mm2.
square kilometer (km2)
a common metric unit of area. One square kilometer equals 100 hectares, 106 square meters, approximately 247.105 acres, or approximately 0.386 102 square mile.
square meter (m2)
the SI unit of area, equal to 104 square centimeters, approximately 10.763 91 square feet, or approximately 1.195 990 square yard.
square mile (mi2 or sq mi)
a traditional unit of area. One square mile is equal to 640 acres, 3 097 600 square yards, 258.9988 hectares, or 2.589 988 square kilometers.
square rod (rd2 or sq rd)
a traditional unit of area. One square rod is equal to 272.25 square feet, 30.25 square yards, or 25.2929 square meters.
square yard (yd2 or sq yd)
a traditional unit of area. 1 yd2 equals 9 ft2, 1296 in2, or exactly 0.836 127 36 m2.
ss
a traditional abbreviation for 1/2, from the Latin semis. In medieval times the symbol was often written as the long s, an obsolete character that looks something like f without the crossbar.
SSU
a symbol for the Saybolt Universal second (see above).
stack (stk)
a traditional unit of volume used in Britain for coal and firewood. The stack equals 4 cubic yards, which is 108 cubic feet or about 3.058 cubic meters. As a firewood measure, a stack equals 27/32 = 0.843 75 cord.
stade, stadion or stadium
a historic unit of distance originating in ancient Greece. Greek athletic fields were all of roughly the same size, and the stadion, equal to 600 podes (feet), was the traditional length of the field. Archaeological measurements show that the stadion was a little more than 200 yards or a little less than 200 meters. The stadion at Olympia, where the original Olympic Games were held, measures 630.8 feet or 192.3 meters; at Athens the stadion was 606.9 feet or 185.0 meters. Stadium is the Latin spelling; in the Roman world the stadium was equal to 625 Roman feet (pes) or 1/8 Roman mile. This is equivalent to 606.95 feet, 202.32 yards, or 185.00 meters. The plural is stadia.
standard (std)
a traditional unit of volume used in northern Europe to measure the volume of finished lumber. The standard equals 165 cubic feet, which is equivalent to 1980 board feet or about 4.672 cubic meters.
standard atmosphere
see atmosphere.
standard cubic meter
see normal cubic meter; also see sccm above.
standard deviation (sd)
a mathematical unit used to describe the "spread" or dispersion of a set of data. Each item in the data set has a deviation from the mean (the ordinary average) of the data. The standard deviation is computed by taking the squares of these individual deviations, averaging these squares, and then taking the square root. If the data set conforms to a known distribution, such as the normal ("bell curve") distribution, then one can compute the percentage of the data which will fall within a certain distance of the mean, as measured in standard deviations. For example, if the data set conforms to the normal distribution, 68.3% of the data will fall within one standard deviation of the mean and only about 4.5% will fall outside 2 standard deviations from the mean.
standard gravity (g or grav)
the average acceleration of gravity at the earth's surface, conventionally defined to be exactly 9.806 65 meters per second per second, or about 32.174 05 feet per second per second. The unit is popularly known as the g or gee.
standard volume
a unit sometimes used by chemists and physicists to measure the volumes of gases. The behavior of gases under ordinary conditions (not at very high pressures or very low temperatures) is governed by the Ideal Gas Law. This law says that the volume V of a gas is related to its temperature T and pressure P by the formula PV = nRT, where n is the number of moles of gas present and the gas constant R equals 8.314 joules per mole per kelvin. One standard volume is the volume one kilomole of gas occupies at standard temperature (273.16 kelvins, or 0 °C) and standard pressure (1 atmosphere, or 101.325 kilopascals). The standard volume is equal to 22.414 cubic meters or 29.316 cubic yards.
stang
a traditional Welsh unit of land area generally equal to 3240 square yards, 0.6694 acre, or 0.2709 hectare.
stanine
a statistical unit used in educational testing. Test scores are normalized (mathematically transformed) so that they have a mean ("average") of 5 and a standard deviation of 2. This tranformation naturally divides the ranked scores into 9 classes called stanines 1-9. The percentage of scores in each stanine is 4, 7, 12, 17, 20, 17, 12, 7, and 4, respectively. The technique originated in the U.S. military during World War II, and the word is a contraction of "standard of nine."
stapp
a unit used to express the effects of acceleration or deceleration on the human body. One stapp represents an acceleration of 1 g for a period of 1 second, or 9.80665 meters per second per second for 1 second. The unit is named for the U.S. Air Force physician John P. Stapp (1910-1999), a pioneer in research on the human effects of acceleration during the 1940s and 1950s.
stat-
a prefix indicating that an electrical unit is part of the CGS electrostatic system. These units are also indicated by the notation esu (as in "volt esu"). Although these units are defined naturally as part of the CGS system, they are usually much too large or much too small for most applications. They have been replaced almost completely by the corresponding SI units. Following are the SI equivalents for each of the "stat" units:
Electric current: 1 statampere = 3.3356 x 10-10 amperes
Electric charge: 1 statcoulomb = 3.3356 x 10-10 coulombs
Capacitance: 1 statfarad = 1.1126 x 10-12 farads = 1.1126 picofarads
Inductance: 1 stathenry = 8.9876 x 1011 henrys
Resistance: 1 statohm = 8.9876 x 1011 ohms
Potential: 1 statvolt = 299.79 volts
Power: 1 statwatt = 10-7 watts
stb
symbol for the stock tank barrel (see below).
stein
a German beer mug. Steins come in various sizes, but the most common size seems to be 1/2 liter (1.057 U.S pint or 0.880 British Imperial pint).
steinkast
see stone's throw (below).
step [1]
a traditional unit of distance, equal to 1/2 pace. The step is traditionally equal to 30 inches or 76.2 centimeters. However, U.S. marching bands often use a shorter step of 22.5 inches (57.15 centimeters), so that 8 steps are made every 5 yards; this works well on American football fields, which have a chalkline every 5 yards. Using this shorter step is called marching "8 by 5."
step [2]
a unit used in music to describe the ratio in frequency between notes. Two notes differ by a step if the higher note has frequency exactly 21/6 = 1.12246 times the frequency of the lower one. This unit is often called the full step to distinguish it from the half step.
steradian (sr)
the standard unit of solid angle measure in mathematics. Just as there are 2pi radians in a circle, there are 4pi steradians in a sphere. Thus one steradian equals about 0.079 577 sphere. There are 129 600/pi = 41 252.96 square degrees in a sphere, so 1 steradian also equals about 3282.806 square degrees. The unit originated in the 1870s by analogy with the radian.
stere (st)
a metric unit of volume, equal to one cubic meter or one kiloliter (about 35.3147 cubic feet or 1.307 95 cubic yards). Although it dates from the origin of the metric system in 1798, the stere has never been used as much as the liter. In Europe it is used primarily as a measure for firewood; for this purpose, the stere equals about 0.2795 cord or 2.207 cord feet. The name comes from the Greek stereos, solid.
sthene (sn)
a metric unit of force, part of the "metre-tonne-second" system sometimes used by European engineers. One sthene is the force required to accelerate a mass of one tonne at a rate of 1 m/s2. Thus the sthene is equal to the kilonewton, so 1 sthene is equivalent to 108 dynes, 224.809 pounds of force or 7233.01 poundals. The name comes from the Greek word sthenos, strength.
stick (stk) [1]
an informal unit used to measure butter. In the United States, butter is usually sold in 1-pound packages containing 4 sticks; thus 1 stick equals 1/4 pound (roughly 113 grams). On the other hand, 1 stick is also considered equal to 1/2 cup (roughly 118 milliliters). Thus the unit is used both as a weight measure and as a volume measure.
stick (stk) [2]
an informal unit of distance used in electrical work in the U.S. One stick is equal to 10 feet (3.048 meters), because electrical conduit of various kinds is traditionally supplied in 10-foot lengths called sticks.
stilb (sb)
a CGS unit of luminance, equal to one candela per square centimeter or 104 nits (candelas per square meter). This is equal to 104pi = 31415.9 apostilbs. The name, believed to have been coined by the French physicist André Blondel (1863-1938) about 1920, comes from a Greek word stilbein meaning "to glitter".
stimp
a measure of the "speed" of a green in golf. The speed is measured by a device called a stimpmeter after its inventor, Edward Stimpson. A golf ball rolls down a ramp inclined at an angle of 20° and then rolls across a level section of the green; the stimp rating is the number of feet the ball travels. A rating of less than 10 stimp is a slow green; championship greens usually have stimp ratings of about 12.
stock tank barrel (stb)
a unit of volume used in the oil industry. A stock tank barrel is the same as the petroleum barrel [2], that is, 42 U.S. gallons or about 158.987 liters, but the oil is required to be at a temperature of 60 °F (15.556 °C). The corresponding metric unit is the standard cubic meter (scm) with the oil at 15 °C. One stock tank barrel is about 0.15899 standard cubic meter.
stoke or stokes (St)
a CGS unit of kinematic viscosity. Kinematic viscosity is defined to be dynamic viscosity (see poise) divided by the density of the liquid; this gives a quantity which depends only on the type of the liquid, independent of its concentration or density. The quotient turns out to have units length2/time. Being a CGS unit, the stokes is therefore defined to be 1 cm2/s, equivalent to 10-4 m2/s or 0.001 076 391 ft2/s. The SI has no named unit of kinematic viscosity, requiring the use instead of m2/s. The unit, called the stokes in Britain and the stoke in the U.S., is named for a British mathematician and physicist, Sir George Gabriel Stokes (1819-1903), who described the basic principles of fluid mechanics in 1845.
stone (st)
a traditional British unit of weight, rarely used in the U.S. Originally the stone varied in size, both from place to place and according to the nature of the item being weighed. A stone of sugar was traditionally 8 pounds, while a stone of wool could be as much as 24 pounds. Eventually the stone was standardized at 14 pounds avoirdupois or approximately 6.350 29 kilograms -- a convenient size because it makes the stone equal to exactly 1/2 (long) quarter [1] or 1/8 (long) hundredweight. In Britain today the stone is commonly used for stating the weight of persons or animals. No -s is added for the plural.
stone's throw
an informal expression in English for "a short distance." 50 meters (164 feet) would be a comfortable stone's throw for most people, although athletes could throw a stone more than twice that distance. The expression may be derived from an old Norse unit, the steinkast (stone throw), for which equivalents between 40 and 60 meters have been listed by various authors.
stop
a unit of relative exposure used in photography. In optics, a "stop" is a ring-shaped baffle or shield used to reduce the aperture (the diameter of the opening) of a lens. In photography, the amount of light used to record the image can be controlled either by varying the aperture, by varying the length of time the shutter is open to admit light, or by some combination of these two methods. Two exposures differ by one stop if one is made with twice the light of the other; similarly, they differ by n stops if one is made with 2n times the light of the other. Since the light transmitted varies with the square of the aperture of the lens, narrowing ("stopping down") the aperture by one stop multiplies the f ratio by the square root of 2. On many cameras, aperture settings are provided, or marked on a continuous scale, with f ratios ("f stops") at successive powers of the square root of 2.
storey or story
an informal unit of distance equal to the average distance between floors of a building. In British English the spelling is "storey" and the plural is "storeys"; Americans write "story" and "stories." Typically a story equals 10 to 12 feet (3.0-3.6 meters) in tall buildings or around 9 feet (2.75 meters) in residential buildings. The origin of this use of the word "story" is not entirely clear, but in medieval times a tier of sculptures or stained glass windows on the front of a cathedral was called a stor(e)y because it usually told a story, and the number of stories was a measure of the size of the building.
STP or stp
an abbreviation for standard temperature and pressure. This notation often appears with statements of the volume of gases; it means the volume measurement is made at or adjusted to a temperature of 0 °C (32 °F) and a pressure of 1 atmosphere, or 101.325 kilopascals.
streck
a unit of angle measurement used in the Swedish military, equal to 1/17.5 degree. This is equivalent to 3.429 minutes of arc (moa), 1.0159 mil (in NATO terminology) or 0.9973 milliradian.
stremma
a traditional Greek unit of area, now redefined to equal exactly 1000 square meters or 0.1 hectare (0.24710 acre).
stride
another name for a pace.
strike
a traditional British unit of volume, varying considerably but usually more than a bushel. A convenient equivalent is 2 Imperial bushels (roughly 2.5 cubic feet or 73 liters).
strob
a metric unit of angular velocity. The strob represents a rotation rate of one radian per second or 9.54930 rpm. The name comes from the Greek strobos, meaning "rotating."
Strong-Cobb Unit (SC)
an ad hoc unit of force previously used to measure the hardness of tablets in the pharmaceutical industry. The Strong-Cobb test machine applied pressure using a hand-operated air pump, and the hardness was read on a gauge marked in 30 arbitrary units called Strong-Cobb units. It is generally believed that 1.4 Strong-Cobb units represented roughly 1 kilogram of force, so each Strong-Cobb unit represented roughly 0.7 kilogram of force or about 7 newtons.
strontium unit (SU)
a unit of radioactive concentration used to measure the presence of the very dangerous radioisotope strontium 90 in the body or elsewhere. In the body or the environment, strontium atoms tend to replace or become mixed with atoms of calcium. One strontium unit is a concentration of 1 picocurie per gram of calcium (pCi/g Ca). In SI units this is equal to exactly 37 becquerels per kilogram (Bq/kg), that is, 37 atomic disintegrations per kilogram per second.
stu
see short ton unit, above.
stubbie
a traditional Australian beer bottle holding 375 milliliters.
stunde
the German word for the hour. In Switzerland, the stunde is also a unit of distance defined to be 4800 meters (2.983 miles) -- approximately the distance a person can walk in an hour. The definition is consistent with the custom everywhere of informally adopting time units as distance units, as when we say "I live 10 minutes from school."
sturgeon
a proposed unit of magnetic reluctance. Reluctance is the opposite of inductance (see henry). The reluctance in sturgeons equals 1 divided by the inductance in henrys; one sturgeon is also equal to one ampere per weber (A/Wb). The unit honors the English engineer William Sturgeon (1783-1850), the inventor of the electromagnet.
Sun or solar mass
a unit of mass used in astronomy to express the masses of stars. Stars are so massive that it's difficult to grasp their relative sizes. Using the Sun's mass as a unit helps us visualize them relative to something familiar. The best current estimate of the mass of the Sun is 1.9891 x 1030 kilograms or 1.9891 x 1027 tonnes; this is about 1048 times the mass of Jupiter and about 332 900 times the mass of the Earth.
superficial
an adjective used to convert a unit of length into a unit of area. "Superficial", in this use, means "square": a superficial foot, for example, is equal to a square foot, and a superficial yard is equal to a square yard. In Australia and New Zealand, a superficial foot often means the super foot [2] (see below).
super foot [1]
a British commercial unit of area equal to one square foot. The name originated as an abbreviation (ft. super. or super. ft.) for the superficial foot.
super foot [2]
in Australia and New Zealand a super foot is a unit of volume for timber or lumber, equal to the volume of a board one foot square and one inch thick. This unit is the same as the North American board foot.
surface foot
another name for a linear foot. See SFM (above).
survey foot
a former U.S. definition of the foot as exactly 1200/3937 meter or about 30.480 060 96 centimeters. This was the official U.S. definition of the foot from 1866 to 1959; it makes the meter equal exactly 39.37 inches. In 1959 the survey foot was replaced by the international foot, equal to exactly 30.48 centimeters. However, the survey foot remains the basis for precise geodetic surveying in the U.S.
SUS
a symbol for the Saybolt Universal second (see above).
suture sizes
suture sizes are stated in both traditional and metric units. The traditional size numbers take into account more than diameter; they consider the tensile strength and other factors as well, so that sutures of the same size perform similarly in the body even if the diameter differs. The traditional sizes are numbers greater than or less than zero, but the negative sizes are indicated by "-0" after the number rather than a minus sign in front of it. Thus the scale from smaller to larger sizes is ...4-0, 3-0, 2-0, ..., 2, 3, 4, ... The metric size number is simply the diameter of the suture in tenths of a millimeter. Since this does not consider tensile strength, surgeons do not have a fixed conversion between traditional and metric sizes.
svedberg (S)
a unit equal to 10-13 second (0.1 picosecond), used to measure sedimentation coefficients in ultracentrifuge technology. The sedimentation coefficient of a particle is defined to be s/(w2r), where s is the sedimentation rate, w is the angular velocity of the centrifuge (in radians per second), and r is the radius at which the particle is being spun; this calculation gives a quantity in time units. The unit honors the Swedish chemist Theodor Svedberg (1884-1971), who received the Nobel Prize in Chemistry for 1926 for the invention of the ultracentrifuge and its use in studying the properties of colloids.
sverdrup
a unit of flow sometimes used in oceanography to express the flow of ocean currents. One sverdrup equals one million cubic meters per second, which is also one cubic hectometer per second. The unit honors the Norwegian oceanographer and Arctic explorer H. U. Sverdrup (1888-1957).
SWG
abbreviation for Standard Wire Gauge. See gauge [3] and the table of wire gauge equivalents.
swing
an informal unit of time describing the length in days of one work cycle. For example, a worker who works 7 days in a row and then gets 3 days off is said to have a swing of 10 days. This usage doesn't seem to be related to the "swing shift," which is a shift that begins in the afternoon and ends at night.
SWU
symbol for the separative work unit, a unit used in the nuclear power industry to describe the work required to enrich uranium (that is, to increase the fraction of uranium 235, the isotope which undergoes fission to produce energy). The details are quite complex, but roughly speaking about 4.5 SWU are needed to enrich 1 kilogram of uranium, more or less depending on the degree of enrichment needed for a particular reactor.
symbol per second (sym/s)
a unit of transmission rate for radio signals, especially for the transmissions between satellites and ground stations. In the simplest form of transmission, data can be sent in bits, which have only two states (on or off). To increase data transmission rates as much as possible, information is sent in units called "symbols", which have a number of states, so that each symbol can include several bits of information. In practice, transmission rates are usually stated in ksym/s (thousands of symbols per second) or Msym/s (millions of symbols per second). See also baud and bit per second.

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September 1, 2004