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

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About the Dictionary
Using the Dictionary


a commercial unit used to measure the effectiveness of thermal insulation. A thermal insulator is a material, manufactured in sheets, that resists conducting heat energy. Its thermal conductance is measured, in traditional units, in Btu's of energy conducted times inches of thickness per hour of time per square foot of area per Fahrenheit degree of temperature difference between the two sides of the material. The R value of the insulator is defined to be 1 divided by the thermal conductance per inch. This means R is an abbreviation for the complex unit combination hr·ft2·°F/Btu. In SI units, an R value of 1 equals 0.17611 square meter kelvins per watt (m2·K/W). In clothing insulation units this is about 1.136 clo or 1.7611 tog. Usually the symbol R is placed before the numerical value, as in R15 or R-15. See also RSI (below).
rad (rd)
a metric unit measuring radiation dose. One rad is equal to a dose of 0.01 joule of energy per kilogram of mass (J/kg), or 100 ergs of energy per gram of mass. The SI unit of radiation dose is the gray (Gy); one rad equals 0.01 gray or 1 centigray. "Rad" is an acronym for "radiation absorbed dose."
radar mile
the time required for a radar signal to travel a distance of one mile from the transmitter to an object, and then return to the receiver. Both ordinary (statute) and nautical miles are used: the radar statute mile is about 10.8 microseconds (µs) and the radar nautical mile is about 12.4 microseconds. A radar kilometer would be about 6.7 microseconds.
radian (rad)
a unit of angle measure widely used in mathematics and science. One radian is the angle at the center of a circle that cuts off an arc of length equal to the radius. Since the circumference equals 2 pi times the radius, one radian equals 1/(2 pi) of the circle, or approximately 57.295 779°. Using radians to measure angles seems unnatural at first. However, when angles are stated in radians the constant pi tends to disappear from the equations, and this greatly simplifies calculation. For example, the length of an arc is simply its radius multiplied by its angular measure in radians, and the area of a sector of a circle is simply its angular measure in radians multiplied by half the square of the radius. The radian was defined and named by James Thomson in 1873. Thomson was a mathematics professor at Queen's College, Belfast, Northern Ireland, and the brother of the famous physicist William Thomson, Lord Kelvin.
radian per second (rad/s)
a common unit of angular velocity. One radian per second is equal to about 9.54930 rpm. This unit has been called a strob.
radiation unit
an older name for the becquerel.
radiocarbon year (14C yr, yr BP)
a unit used in stating the nominal ages of plant or animal remains dated by radiocarbon testing. A very small proportion (roughly 1 part per trillion, or 10-12) of the carbon in the ecosystem is radioactive carbon-14, which decays to nitrogen-14 with a half life of about 5760 years. While a plant or animal is alive, the fraction of radioactive carbon in its body remains equal to the fraction in the atmosphere at that time. After a plant or animal dies, as the carbon-14 trapped in its body slowly decays, the age of the tissue can be measured by the fraction of radioactive carbon remaining. The age T of a sample, in radiocarbon years, is computed from the formula T = -8033 ln (R/A), where R is the measured ratio of carbon-14 to ordinary carbon-12 in the sample and A is a benchmark ratio measured in the atmosphere in 1950. Results are often stated as years before present (yr BP), with 1950 chosen to be the "present." The results are inaccurate for several reasons. Obviously, 1950 is no longer the present. Another problem is that the formula assumes a half life of 5568 years, which is now known to be too short (the actual value is 5730±40 years). Most importantly, the ratio of carbon-14 to ordinary carbon-12 in the atmosphere varies slightly over time. Much research has been done to determine the necessary corrections. As an example, a sample with a nominal age of 12 000 radiocarbon years has an actual age of about 14 000 years. A table is provided, and a technical report (pdf document) posted by the University of Arizona has full details.
a traditional unit of land area in Thailand. The rai is now considered to equal exactly 1600 square meters, which is 0.16 hectare or approximately 0.3954 acre. The rai is divided into 4 ngan. The unit is called the hai in northern Thailand and the lai in Laos. The word means "field," that is, an upland field rather than a rice paddy.
an absolute temperature scale; see degree Rankine.
see rotl (below).
one of two units of sound impedance. When sound waves pass through any physical substance the pressure of the waves causes the particles of the substance to move. The sound impedance is the ratio between the pressure and the particle velocity it produces. The impedance is 1 rayl if unit pressure produces unit velocity. In MKS units, this means 1 rayl equals 1 pascal-second per meter (Pa·s/m), or (equivalently) 1 newton-second per cubic meter (N·s/m3). Confusingly, the same name, rayl, is used for the corresponding CGS unit, 1 dyne-second per cubic centimeter (dyn·s/cm3). The CGS rayl equals 10 MKS rayls. The units are named for Robert John Strutt, the fourth Lord Rayleigh (1875-1947).
rayleigh (R)
a CGS unit of light intensity used in astronomy and physics to measure the brightness of the night sky, auroras, etc. One rayleigh represents the light intensity of one million photons of light emitted in all directions per square centimeter of receiver per second, or, in SI units, 795.775 x 106 per square meter per steradian (m-2·sr-1). A dark night sky has a light intensity of roughly 250 rayleighs. The unit honors the English mathematician and physicist John William Strutt, the third Lord Rayleigh (1842-1919).
ream (rm) (1)
a traditional unit of quantity used for counting sheets of paper. The word is thought to be derived from the Arabic rizmah, meaning a bundle. A ream is equal to 20 quires, which would be 480 sheets with the traditional definition of a quire as 24 sheets. In recent years, however, the ream has been redefined to equal 500 sheets. (Working backwards, this changes the definition of a quire from 24 to 25 sheets.) The new definition reflects the current practice of marketing many kinds of paper in packages of 500 sheets. The older size of 480 sheets is now called a short ream.
ream (rm) (2)
a traditional unit of area equal to 3000 square feet (about 278.709 square meters). This unit represents the total area of a ream (1) of 500 full-size sheets of paper, each sheet being 3 feet by 2 feet. The "area ream" is used commonly in the U.S. paper industry for kraft paper, paperboard, and similar products, and it is also being used for non-paper products such as window films and other plastic films.
a temperature scale; see degree Réaumur.
an ancient Hebrew unit of weight or mass equal to 1/4 shekel. The word means "quarter" in Hebrew.
rebar sizes
numerical size designations for steel reinforcing bars ("rebars") used to strengthen concrete. The size number is the diameter of the rod in 8ths of an inch (1/8 inch = 3.175 millimeters); thus a rod 1 inch in diameter is a #8 rebar.
reciprocal megakelvin (MK-1)
a unit used in colorimetry and photography to measure the wavelength of light, especially for selecting filters to adjust the "color temperature." Light waves of a specific wavelength w meters can be assigned a temperature T using the theory of blackbody radiation. A blackbody is an ideal object that absorbs all the radiation it receives. If a blackbody is heated to temperature T, the radiation it gives off will have maximum intensity at wavelength w, where w and T are related by Wien's law, wT = 2.90 x 10-3 meter kelvins. The reciprocal of temperature, 1/T = w/(2.90 x 10-3) K-1, is thus a measure of wavelength. The wavelengths of visible light fall in the range 400 nanometers (extreme violet) to 700 nanometers (extreme red), corresponding to reciprocal temperatures in the range from roughly 130 MK-1 to 240 MK-1. The reciprocal megakelvin has also been called the mired.
recommended dietary allowance (RDA)
units used in the U.S. to measure the amounts of certain nutrients found in foods or provided by supplements such as vitamin tablets. Each nutrient has its own RDA unit. Link: Dietary Reference Intakes from the University of Texas.
redshift (z)
a unit of relative distance used in astronomy. The universe is expanding, so distant galaxies are receding from the earth. The faster the speed of recession, the farther the object. Just as sound from a receding train is lowered in pitch, light from distant galaxies is shifted toward longer wavelengths, that is, toward the red end of the spectrum. The redshift equals z if the wavelength of light is z + 1 times the normal wavelength; thus a redshift of 0.40 means that the wavelength of the light is 40% longer than normal. Using the Hubble space telescope, astronomers have measured redshifts greater than 5.0.
Redwood second
an obsolete unit of kinematic viscosity given by readings on the Redwood viscometers commonly used in Britain and elsewhere. The reading is the time, in seconds, for 50 milliliters of a sample of a liquid to flow through the device. The viscosity in centistokes is given roughly by the formula 0.260 t - (0.0188 / t ), where t is the flow time in seconds.
register ton (RT)
a unit of cargo capacity equal to 100 cubic feet (about 2.832 cubic meters); see ton [3]. The symbol RT seems to be in wide use for this unit, but it is also used for the refrigeration ton (see ton [7]) and for the revenue ton (see below).
a large wine bottle holding about 4.5 liters, 6 times the volume of a regular bottle. The "h" is silent in English pronunciation.
a unit used for measuring the effective (or "equivalent") dose of radiation received by a human or some other living organism. One rem is equal to 0.01 sievert (Sv) or 10 millisieverts, which means it equals the actual dose received in rads (see above), multiplied by a "quality factor" which is larger for more dangerous forms of radiation. The rem is related to the rad in the same way that the sievert is related to the gray. "Rem" is an acronym for "roentgen equivalent: man," meaning that it measures the biological effects of ionizing radiation in humans. The unit was introduced in 1944 by the physicist H.M. Parker of the Manhattan Project.
an obsolete unit of absorbed radiation dose equal to the absorption of 93 ergs of energy per gram. This is equivalent to 0.93 rad (see above) or 9.3 milligrays (mGy). "Rep" is an acronym for "roentgen equivalent: physical." The definition was made because a dose of 1 rep of beta rays was considered biologically equivalent to a dose of 1 roentgen (see below) of X rays.
reputed pint
see pint [4].
reputed quart
see quart [2].
res or RES
symbol for "resolution," a unit defined to be the number of dots or pixels per millimeter in an image. The unit is often stated before the measurement. RES 1 is equal to 25.4 dots per inch (dpi).
retinol equivalent (RE)
a unit of dosage for retinol (vitamin A) and for related substances such as beta carotene. One RE is equivalent to 5 international units (IU), or 1.5 micrograms, of retinol. U.S. nutritional authorities recommend that an adult diet provide 1000 RE per day.
revenue ton or tonne (RT)
a unit used for billing in the shipping industry. The size of a shipment in revenue tons is the number of metric tons or the number of cubic meters in the shipment, whichever is larger.
revolution (r or rev)
a unit of angle measure equal to a full circle, 360°, or 2 pi radians.
revolution per minute (RPM or rpm or r/min)
a unit of angular velocity, used particularly for rotation rates in mechanics. One r/min equals 0.104 720 radian per second. The tachometers on auto dashboards are usually calibrated in units of 1000 r/min.
a unit of dynamic viscosity in the customary English system. See poise for a description of dynamic viscosity. With force measured in pounds of force (lbf), one reyn equals 1 lbf·s/in2, approximately 68.947 57 kilopoise, or 6.894 757 kilopascal second. The unit, pronounced "ren", was named for a British scientist, Osborne Reynolds (1842-1912).
a unit of fluidity, the opposite of viscosity. The unit, pronounced "ree", was introduced by the American chemist F.C. Bingham in 1928; he defined it as the reciprocal of the centipoise. However, it came to be used instead as the reciprocal of the poise itself, so the fluidity of a substance in rhes is 1 divided by its dynamic viscosity in poise. In SI units, the rhe equals 10 per pascal-second (10 (Pa·s)-1). The name of the unit comes from the Greek rhein, to flow.
Rhine foot or Rheinfuss
the foot unit, or fuss, traditionally used in western and northern Germany.
a unit used in physics to measure the strength of gamma rays, a form of high-energy radiation emitted by some radioactive substances. A source of strength 1 rhm produces ionization at the rate of 1 roentgen per hour at a distance of 1 meter from the source. The letters "rhm" stand for roentgen-hour-meter.
Rhode Island
the smallest state of the U.S., Rhode Island has long served as an informal unit of area in statements such as "an iceberg 1.5 times the size of Rhode Island has broken off from Antarctica." Rhode Island has a land area of about 1045 square miles or 2706 square kilometers. Europeans might note that Luxembourg (2586 square kilometers) provides a comparable unit. See also Wales.
a traditional Japanese unit of distance, sometimes called the Japanese league because it is of similar length to the European league. The ri equals 2160 ken or 12 960 shaku (the shaku being the Japanese equivalent of the foot). This is about 3927 meters or 2.44 statute miles.
rice cup
a common name in English for the Japanese go, a unit of dry and liquid capacity equal to about 180 milliliters or 3/4 of a U.S. cup. Japanese rice cookers usually come with measuring cups having this capacity.
Richter scale
a logarithmic scale measuring earthquake intensity. See magnitude [2].
a traditional unit of volume for firewood. A rick represents a stack of split firewood 4 feet high and 8 feet long, the logs being of a standard length, usually 16 inches. This is equivalent to 1/3 cord or 1.208 steres. However, because the size of a rick has been manipulated by vendors, it is illegal to sell firewood by the rick in several U.S. states. A rick is sometimes called a face cord or tier. The name of the unit comes from a old Norse word for a stack of wood.
a traditional Welsh unit of distance equal to 3 leaps or 20 feet 3 inches (6.1722 meters).
Riga last
a traditional British unit of volume used for measuring timber. The Riga last is named for the Latvian capital, Riga, which was a major port for the shipment of timber from Russian forests. A Riga last is 80 cubic feet (2.265 cubic meters) of square-sawn timber or 65 cubic feet (1.841 cubic meters) of round timber. See also last.
right angle
a common unit of angle measure equal to 1/4 circle, 90°, 100 grads, or pi/2 radians.
a traditional English unit of quantity for boards and staves, which were shipped encircled by metal rings. A ring equals 4 shocks, or 240 boards.
ring size
a measure of the inside diameter or inside circumference of a ring (the kind worn on a finger). A variety of ring sizing systems are used in various countries. In the U.S., a ring of size n has an inside circumference of 1.43 + 0.102·n inches, or about 36.3 + 2.60·n millimeters. (There is some variation, because U.S. ring sizes have never been standardized). In Britain, traditional ring sizes are stated as letters A, B, etc.; if we replace the letters by numbers n (A = 1, B = 2, etc.), then a ring of British size n has an inside circumference of 36.25 + 1.25·n millimeters, or about 1.43 + 0.049·n inches. A difference of 1 U.S.size thus corresponds rather closely to two letters in the British system. In Japan, sizing is by the inside diameter in increments of 1/3 millimeter; a ring of Japanese size n has an inside diameter of (n + 38)/3 millimeters and an inside circumference of 39.8 + 1.047·n millimeters. There is an international standard (ISO 8653) defining the ring size to be the inside circumference in millimeters, minus 40. Rings are now sized by this standard in most of Europe, so a ring of European size n has an inside circumference of exactly 40 + n millimeters. (The British scale is aligned with the European scale, with British size C corresponding to European size 0 and a difference of four British letters corresponding to 5 European sizes.)
rms or RMS
an abbreviation for root mean square (see below), a mathematical technique for averaging the values of a changing quantity.
Rockwell hardness (RH-)
a measure of the hardness of a metal introduced by Rockwell in 1922. In a Rockwell hardness test, a penetrator makes an indentation in the metal under two constant loads, a "minor" load (generally 10 kilograms) and then a "major" load. The difference in penetration depth between the two loads provides the measure of the hardness, usually read from a gauge on the testing machine. There are several Rockwell scales for different ranges of hardness. The most common are the B scale (RHB), for which a steel ball is used as the penetrator, and the C scale (RHC), for which a cone-shaped diamond is used. The B scale is appropriate for soft metals, the C scale for hard metals. Rockwell hardness numbers are not proportional to Brinell or Vickers hardness readings.
rod (rd) [1]
a traditional unit of distance equal to 5.5 yards (16 feet 6 inches or exactly 5.0292 meters). The rod and the furlong were the basic distance units used by the Anglo-Saxon residents of England before the Norman conquest of 1066. The Saxons generally called this unit the gyrd, a word which comes down to us as the name of a different unit, the yard. "Rod" is another Saxon word which meant just what it means today: a straight stick. The Normans preferred to call the gyrd a pole or a perch (a word of French origin, meaning a pole; see perche). The length of the rod was well established at least as early as the eighth century. It may have originated as the length of an ox-goad, a pole used to control a team of 8 oxen (4 yokes). Scholars are not sure how the rod was related to shorter units. It may have been considered equal to 20 "natural" feet (actual foot lengths; see foot), or it may have been measured "by hand" as 30 shaftments. In any case, when the modern foot became established in the twelfth century, the royal government did not want to change the length of the rod, since that length was the basis of land measurement, land records, and taxes. Therefore the rod was redefined to equal 16.5 feet, because with reasonable precision that happened to be its length in terms of the new foot. This length was called the "king's perch" at least as early as the time of King Richard the Lionheart (1198). Although rods and perches of other lengths were used locally in Britain, the king's perch eventually prevailed. The relationship between the rod and the other English distance units was confirmed again by the Parliamentary statute of 1592, which defined the statute mile to be either 320 rods or 1760 yards, thus forcing the rod to equal exactly 5.5 yards or 16.5 feet.
rod (rd) [2]
a unit of area equal to one square rod [1]. A rod of area covers exactly 272.25 square feet or about 25.292 85 square meters. There are 40 rods in a rood and 160 rods in an acre.
a traditional Dutch unit of distance (see rood [1] below), reinterpreted in 1820 as a metric unit equal to exactly 10 meters (32.8084 feet). The roede has also been used as a unit of area equal to one square (linear) roede; this is equal to 100 square meters or 1 are.
roentgen or röntgen (R)
a non-metric unit used to measure the ionizing ability of radiation. Radiation often ionizes atoms it strikes, stripping one or more electrons from them. The biological effects of radiation are caused in large part by excessive ionization within living cells, so it is important to measure this ionizing ability of radiation. For x-rays and gamma rays, this is often done by measuring the electric charge released when air is ionized by the radiation. The roentgen is an old unit used for this purpose; one roentgen equals a charge release rate of 258 microcoulombs per kilogram of air. The unit is named for one of the early investigators of radioactivity, the German physicist Wilhelm Konrad Röntgen (1845-1923). In English it is usually pronounced "rent-gen" with a hard g sound; sometimes the soft g ("rent-jen") is used.
rood [1]
an old unit of distance, used in several ways. Rood (or roede) is an old Dutch word meaning a rod or pole. So the rood is in some cases another name for a rod [1]. But in old England and Scotland the rood was often longer than a "modern" rod of 16.5 feet; sometimes it was 20 feet, 21 feet, or even 24 feet. In Afrikaans-speaking South Africa, the rood was a standardized measure equal to 12 Cape feet, which is 12.396 English feet or 3.7783 meters.
rood [2]
a traditional unit of area used to measure land. A rood is the area of a narrow strip of land one furlong (40 rods, or 660 feet) long and one rod (16.5 feet) wide. Thus the rood is equal to 40 square rods (or perches), which equals 1210 square yards, or 10 890 square feet, or exactly 1/4 acre. That would be the area of a lot 22 yards wide and 55 yards deep, about the size of many suburban lots. One rood is approximately 1011.714 square meters, or 0.101 171 4 hectare.
root mean square (rms)
a notation used after various measurements to indicate that the root mean square method has been used to measure or compute an average value for the measurement. Usually the quantity being measured varies in a periodic way; typical examples include the voltage of an alternating current or the intensity of a sound wave. In the rms method, the varying quantity is first squared (S), then a mean (M) or average of the squared values is obtained, and then the square root (R) of this mean value is computed. For many purposes this procedure gives the best measure of the "typical" or "effective" value of the quantity.
abbreviation for research octane number. See octane number.
a unit of land area in Nepal, equal to about 508.7 square meters or 5475 square feet (0.05087 hectare or almost exactly 1/8 acre). The ropani is divided into 16 annas.
another name for the rood [1], the distance unit.
rotl, rotel, rottle, ratel, or arratel
a traditional Arab unit of weight corresponding to the Roman libra, the French livre, and the English pound. There was considerable variation in the unit from time to time and from place to place, but usually the rotl was about 0.9-1.15 pound (450-530 grams). However, in some areas of the Near East, such as Syria and Palestine, larger rotls of 5.5 to 6 pounds (2.5-2.8 kilograms) were used. This unit has many spellings in European languages.
the basic unit of time in boxing, equal to 3 minutes.
royal foot
the French pied de roi, also called the Paris foot in English.
a very common abbreviation for revolutions per minute (see above).
a symbol for the R-value of insulation when stated in SI units: square meter kelvins per watt (m2·K/W). RSI 1 is equivalent to R 5.678.
a unit of density for woolen yarn, used in the U.S. Yarn is described as n run if there are n 1600-yard hanks of the wool per pound. Actual yarn ranges from about 0.5 run to 8 run. The unit is also called the American run.
a traditional measure of liquid volume, dating back to the Middle Ages. A rundlet is a small barrel usually holding 18 wine (U.S.) gallons (roughly 68.1 liters).
running foot
another name for a linear foot. Terms such as running meter and running yard are used similarly.
a traditional German unit of distance corresponding to the English rod [1]. In fact, "rute" is the German word for rod. The rute had varying lengths, as short as 10 fuß (German feet) and as long as 16 fuß. This could be anywhere from about 3 meters to 4.5 meters.
rutherford (Rd)
a practical unit of radioactivity equal to the megabecquerel (MBq). This means 1 rutherford represents 1 million radioactive disintegrations per second. The unit is named for the New Zealand nuclear physicist Ernest Rutherford, later named Lord Rutherford (1871-1937), whose study of radioactivity led to the discovery in 1911 that most of the mass of an atom is concentrated in a tiny nucleus.
rydberg (1)
a former name for the kayser, the CGS unit of wave number. The name honored the Swedish physicist Johannes Rydberg (1854-1919).
rydberg (2)
a unit of energy used in atomic and molecular physics, equal to about 2.1799 x 10-18 joule or 13.60569 electronvolts. The rydberg is also equal to exactly 0.5 hartree.

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Checked and revised April 25, 2002