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


P or p
a common short form of PM or pm (see below), used in statements of time.
paardekracht (pk)
the Dutch word for the metric horsepower.
pace [1]
a traditional unit of distance equal to the length of a person's "full" pace, that is, the distance between two successive falls of the same foot. Thus one pace equals two steps. The Romans counted 1000 paces in a mile with each pace being a little over 58 inches (or about 148 centimeters). In English speaking countries, the pace is usually defined to be exactly 5 feet (or 152.4 centimeters); this unit is also called the great pace or geometrical pace. Obviously, a good metric version of the pace is exactly 1.5 meters.
pace [2]
in military use, the term "pace" is often used as an alternate name for the step; see military pace.
pack (pk)
a commercial unit specifying the number of items per package. In retail trade, packages containing 4 items (for example) are often described as "4 pack" or "4pk." The symbol pk is also used for the peck (see below).
pack year
a unit of quantity for cigarettes used in medicine to measure a patient's smoking history. One pack year is the equivalent of smoking one 20-cigarette pack per day for one year, that is, a total of 7300 cigarettes.
a traditional Russian unit of weight equal to 1200 funte, 30 pudi (see pud below), 1083 pounds, or 491.4 kilograms.
pair (pr)
a unit of quantity equal to 2. The word is from the Latin paria, meaning "equals." Originally a pair was simply a group of similar objects, number unspecified. Eventually this meaning was specialized to refer to a group of two.
pair royal
a unit of quantity equal to 3, used in cribbage to describe three cards of the same rank. This usage recalls the original meaning of "pair" as a group of equivalent objects, not necessarily two in number. Four cards of the same rank form a "double pair royal."
Palermo scale
a scale used by astronomers to assess the risk of an impact on the earth by a comet or asteroid. The scale value is a logarithmic measure of the risk of impact compared to the average risk of an impact by objects of the same size or larger over the years until the date of the potential impact by the newly discovered object. If the object in question has probability p of impact at a time T years in the future, the Palermo scale value is PS = log10 (p/(0.03TE-4/5)), where E is the projected energy of the impact in megatons of TNT. A possible impact is considered to be of concern if the Palermo scale value exceeds -2, that is, if the impact is more than 1% as likely as a random impact. The scale is used to prioritize the need for further observations of an object. See also Torino number.
palm [1]
a traditional unit of distance equal to the width of a person's palm. The palm equals 4 digits or 1/6 cubit, which is about 3 inches or 7.5 centimeters. This unit was used very commonly in medieval and early modern Britain. Similar units, all equal to 1/4 the local "foot" unit, were used throughout northern Europe.
palm [2]
a traditional unit of distance equal to the length of a person's hand, from the wrist to the end of the middle finger. In the English system this unit is equal to 9 inches (22.86 centimeters) and is usually called a span. The confusion between the two palm units is ancient. In Roman times, the longer unit was known as the palmus major and the shorter one as the palmus minor. In the nineteenth century, the 3-inch version was more common in Britain and the 9-inch version was more common in the U.S., perhaps because some Americans were familiar with the comparable Spanish palmo (see below).
palm [3]
a name sometimes used in Dutch for the decimeter (10 centimeters, or about 3.937 inches).
an informal unit of volume popularized recently by Rachael Ray on her television shows in the U.S. Ray uses the palmful to mean a tablespoon (15 milliliters).
a traditional unit of distance in Spain and Portugal. The traditional Spanish palmo equals 9 pulgadas (see below) or 1/4 vara: this is about 20.9 centimeters in Spain and a little more than that in Spanish Latin America. In Texas, 1/4 vara comes to 8 1/3 inches (21.17 centimeters). Under the metric system in Spain, the palmo is an informal unit equal to 20 centimeters. The Portuguese palmo equals 0.1 braça or about 22.0 centimeters (8.66 inches). These units are based on the width of a person's outstretched hand, from the tip of the thumb to the tip of the little finger, a definition identical to that of the English span.
a historic unit of distance comparable to the European league. The unit originated in Persia but was used throughout the ancient Middle East and Mediterranean. It was equal to roughly 3.5-4.0 miles or about 6 kilometers. In Arabic the unit is called the farasang.
Paris foot
an English name for the French royal foot (pied de roi, see below).
Paris point
a unit of length equal to 2/3 centimeter (0.2625 inch) used to measure shoe size in most European countries.
parsec (pc or psc)
a non-metric unit of distance used in astronomy. As the Earth makes its orbit around the Sun, nearby stars appear to shift their positions in relation to the background of distant stars. This shift, called the parallax of the star, is very small, less than one arcsecond even for the nearest stars. One parsec is the distance at which a star would appear to shift its position by one arcsecond during the time in which the Earth moves a distance of one astronomical unit (au) in the direction perpendicular to the direction to the star. Using this unit makes it easy to compute distances: the distance to a star, in parsecs, is simply one divided by the parallax, in arcseconds. If the parallax is 0.01 arcsecond, the distance is 100 parsecs. One parsec divided by one astronomical unit (the length of the semimajor axis of the Earth's elliptical orbit) is the trigonometric function of 0.01 arcsecond called the cotangent; from this relation we can compute that one parsec equals 206 264.8 au. This is the same as about 3.261 56 light years, 30.856 78 petameters (30.856 78 x 1012 kilometers), or 19 173 510 000 000 miles.
part [1]
a unit used in informal statements of proportion or in prescriptions for mixtures. The fraction of an ingredient present is the number of parts of that ingredient divided by the total number of parts present. For example, hummingbirds can be attracted to feeders with a mixture of 1 part sugar dissolved in 4 parts water (equivalent to a solution of 1/5 sugar and 4/5 water, by volume).
part [2]
a medieval unit of time equal to 1/15 hour or 4 minutes.
part [3]
in calculations involving the Jewish calendar, an informal name for the helek, a unit of time equal to exactly 10/3 seconds. There are 1080 parts in an hour.
particle flux unit (pfu)
a unit used to measure the rate at which energetic particles, such as protons, are received by spacecraft. These flux rates are a major component of the "space weather," the environment in which satellites and other spacecraft operate. One pfu is a rate of one particle per square centimeter of detector area per steradian of solid angle scanned per second of time. (A steradian is about 7.96% of a sphere.) In SI units, 1 pfu = 10-4 m-2sr-1s-1.
part per billion (ppb)
a unit of proportion equal to 10-9.
part per million (ppm)
a unit of proportion equal to 10-6.
part per quadrillion (ppq)
a unit of proportion equal to 10-15.
part per thousand (ppth or ppt)
a unit of proportion equal to 0.001, also called per mill (see below).
part per trillion (ppt)
a unit of proportion equal to 10-12.
pascal (Pa)
the SI unit of pressure. The pascal is the standard pressure unit in the MKS metric system, equal to one newton per square meter or one "kilogram per meter per second per second." Sounds impressive, but in traditional English terms a pascal is only 0.000 145 pounds per square inch (0.020 885 lbf/ft2 or 0.007 50 mmHg). Thus pressure is more commonly measured in kilopascals (kPa), with 1 kPa = 0.145 lbf/in2. Air pressure is also measured in hectopascals (hPa), with 1 hPa = 1 millibar. The unit is named for Blaise Pascal (1623-1662), French philosopher and mathematician, who was the first person to use a barometer to measure differences in altitude.
pascal second (Pa·s)
the SI unit of dynamic viscosity, equal to 10 poises or 1000 centipoises. Some scientists propose calling this unit the poiseuille (Pl), but that name has not been accepted by the General Conference on Weights and Measures.
an individual serving of butter. In the U.S. food industry, restaurant servings of butter were traditionally packaged at 48 pats per pound, making each pat 1/3 ounce (about 9.45 grams). Less-generous portions such as 60, 64, or 72 pats per pound are also available (margarine is often supplied in these smaller portions). Outside the U.S., butter is traditionally packaged at 100 pats per kilogram, making each pat 10 grams, but packages of 125 per kilogram (8-gram pats) or 150 per kilogram (6.67-gram pats) are also available. In U.S. recipes, a pat of butter is typically 2 teaspoons (1/3 fluid ounce, or about 10 milliliters).
pcf, pci
symbols for pound per cubic foot or per cubic inch, traditional engineering units of density. 1 pcf = 16.018 46 kg/m3 and 1 pci = 1728 pcf = 27 679.90 kg/m3.
the traditional Portuguese foot, equal to 12 polegadas or about 33.324 centimeters (13.12 inches).
pearl grain
a unit of mass equal to 1/4 carat or 50 milligrams; see grain [2].
pebi- (Pi-)
a binary prefix meaning 250 = 1 125 899 906 842 624. This prefix, adopted by the International Electrotechnical Commission in 1998, is supposed to replace peta- for binary applications in computer science. The prefix is a contraction of "petabinary."
peck (pk)
a traditional unit of volume, formerly used for both liquids and solids but now used mostly for dry commodities such as grains, berries, and fruits. A peck is 2 gallons, 8 quarts, or 1/4 bushel. In the U. S. customary system, a peck holds 537.605 cubic inches or approximately 8.8098 liters. In the British Imperial system, a peck is a little larger, holding 554.84 cubic inches or approximately 9.0923 liters. In Scotland, the traditional peck held about 9.1 liters for wheat, peas, or beans and about 12.1 liters for barley or oats. The word "peck", originally spelled "pek", comes from the name of a similar old French unit; the origin of the French unit is not known.
pencil hardness
a traditional measure of the hardness of the "leads" (actually made of graphite) in pencils. The hardness scale, from softer to harder, takes the form ..., 3B, 2B, B, HB, F, H, 2H, 3H, 4H, .... The letters stand for Black, Hard, and Firm. (There is no industry standard defining the scale, so there is some variation between manufacturers in how it is applied.) In the U.S., many manufacturers use a numerical scale in which the grades B, HB, F, H, 2H correspond approximately to numbers 1, 2, 2-1/2, 3, and 4, respectively. The pencil hardness scales are not just used for pencils, however. They are used widely to state the durability of paints and other semi-soft coatings. The hardness rating of a coating is the hardness of the hardest pencil that does not penetrate and gouge the coating. This "scratch" hardness scale is analogous to the well-known Mohs hardness scale used in geology to measure the hardness of minerals.
-penny (-d)
an ending added to a number to indicate the size of a nail, as in "sixpenny (6d) nail" or "tenpenny nail." It's not clear exactly how this terminology began, although the usual guess is that tenpenny nails originally cost ten pence per hundred. There is, very roughly, a linear relation between the size designations and length: an n-penny nail is roughly (1/2) + (1/4)n inches long. This makes the tenpenny nail about 3 inches long, the eightpenny about 2.5 inches, and so on.
in traditional British usage, another name for part [1] as used with sums of money. An expression such as "the fourth penny" means 1/4 of the sum.
pennyweight (dwt or pwt)
a unit of weight in the traditional troy system (see also pound [2]), equal to 24 grains or 1/20 troy ounce. One pennyweight is approximately 1.5552 gram. The d in the traditional symbol dwt is from the Latin word denarius for the small coin which was the Roman equivalent of a penny. (The letter d was also the symbol for the penny in the traditional English monetary system.) See troy weights for additional information.
pentad [1]
a unit of quantity equal to 5.
pentad [2]
a unit of time equal to 5 days. This unit is common in meteorology, where forecasts are frequently made for periods of 5 days at a time.
per annum (PA)
a traditional unit of frequency equal to once a year.
percent or per cent or per centum (% or pct or pc)
a unit of proportion, equal to 0.01. The word is Latin, meaning "by the hundred." The symbol % can be placed after any number; mathematically, its effect is an immediate division by 100.
percentage point
a unit of proportion, equal to 0.01 or 1%. This unit is used commonly to describe changes in rates or other quantities that are stated as percentages. For example, an interest rate that rises from 8% to 10% is said to rise by two percentage points. Notice that if the changing rates are percentages of a quantity y then the percentage points are also percentages of y. This is different from looking at the proportional change in the rates themselves: in our example, an increase from 8% to 10% is an increase of one-fourth (25%) in the actual interest rate. It often happens that a change of a few percentage points has a dramatic effect on the rate in question.
a unit used in statistics to describe a portion of the individuals or events being studied. Suppose the data are arranged by numerical scores, from highest to lowest. A score belongs to the 78th percentile, for example, if it is greater than 78% of the scores but it is not greater than 79% of the scores. This procedure divides the scores into 100 percentiles, numbered 0th through 99th.
perch [1]
an alternate name for the rod [1] (16.5 feet or 5.0292 meters), introduced in the twelfth century by the Norman conquerors of England. The word perch (perche in French: see below) comes from the Latin pertica (pole). The Romans also had a distance unit called the pertica, but it was shorter: 10 Roman feet (9.71 English feet or 2.96 meters).
perch [2]
a unit of area equal to one square perch [1]. A perch of area covers exactly 272.25 square feet or about 25.292 85 square meters. There are 40 perches in a rood and 160 perches in an acre. Land in Sri Lanka is measured customarily in perches.
perch [3]
a traditional unit of volume for stone and other masonry. A perch of masonry is the volume of a stone wall one perch (16.5 feet) long, 18 inches high, and 12 inches thick. This is equivalent to exactly 24.75 cubic feet, 0.916 667 cubic yard, or about 0.700 842 cubic meter.
perch [4]
a traditional unit of distance in Ireland standardized at 21 English feet (6.4008 meters) or 14/11 English perch or rod. Since the Irish perch was 27.27% longer than the English, the Irish chain, furlong, and mile were longer in the same proportion..
perche [1] or perch [5]
a traditional unit of distance in French North America. The perche equals 18 pieds (see below) or 3 toises. By legal definition in Canada this equals 19.1835 English feet or 5.847 13 meters.
perche [2] or perch [6]
a traditional unit of area in French North America and also in Mauritius, equal to one square perche [1]. A perche of area is therefore equal to 0.01 arpent, about 368.007 square feet (40.8896 square yards) or 34.189 square meters.
per diem (PD)
a traditional unit of frequency equal to once a day.
perfect ream
see printer's ream (below).
a traditional unit of water vapor permeability, that is, the ability of a material to permit the passage of water vapor. If we want to keep things dry, we wrap them in something having low permeability. A material has a permeability of one perm if it allows transmission of one grain of water vapor per square foot of area per inch of mercury (inHg) of pressure difference per hour. The value depends somewhat on temperature, however. At 0 °C, one perm equals about 5.721 x 10-11 kilograms per square meter per pascal per second (kg/(m2·Pa·s)) or about 0.2060 mg/(m2·Pa·h); at room temperature the equivalent is about 5.745 x 10-11 kg/(m2·Pa·s). [The SI unit, kg/(m2·Pa·s), simplifies to seconds per meter (s/m).] The lower the perm value, the better the vapor barrier.
per mensem
a traditional unit of frequency equal to once a month.
per mill, per mil, or per mille
a unit of proportion, equal to 0.001 or 1 per thousand. Unlike percent, per mill is usually written as two words, although the one-word spellings permill and permil are also used. Its symbol, not available to most web browsers, is like the percent symbol but with two zeroes in the denominator (roughly, 0/00). The spelling "per mill" seems to be more common in the U.S., "per mil" being more common in Britain.
perm inch
a traditional unit of water vapor permeance. The perm value (see above) does not depend on the thickness of the material used as a water barrier. The permeance is the product of the perm value and the thickness, measured in inches. One perm inch equals about 1.453 x 10-12 kg/(m·Pa·s) at 0 °C or about 1.459 x 10-12 kg/(m·Pa·s) at room temperature. The SI unit kg/(m·Pa·s) actually simplifies to seconds (s).
person hour
a gender-nonspecific version of man hour, a unit of labor equal to the work of one person for one hour.
another name for per mill (see above). In typography, "perthousand" is often used as the name of the per mill character.
the Roman foot, equal to 29.67 centimeters (about 11.68 inches). The pes was divided into 12 unciae (inches). There were 5 pes (or pedes) in 1 passus (pace, see above), 10 in a decempeda, 625 in a stadium, and 5000 in the Roman mile.
peta- (P-)
a metric prefix denoting 1015 (one U.S. quadrillion). One parsec, for example, equals 30.857 petameters. The prefix was chosen to suggest the Greek penta, meaning 5, this being the fifth prefix (n = 5 in 103n) in the SI system of metric prefixes. The prefix is usually pronounced pet-a, with a short "e" sound, rather than pee-ta.
petabecquerel (PBq)
a unit of radioactivity equal to 1015 atomic disintegrations per second or 27 027.03 curies.
petaflops (Pflops)
a unit of computing power equal to one quadrillion (1015) floating point operations per second. See flops. Current computers cannot achieve this power; it is a goal for future generations of supercomputers.
petagram (Pg)
a metric unit of mass equal to 1015 grams or 1 gigatonne (one billion metric tons). This unit is used in atmospheric science and other scientific contexts where large masses are considered.
petahertz (PHz)
a unit of frequency equal to 1015 hertz. The frequencies of infrared and visible light waves are expressed in petahertz.
petajoule (PJ)
a metric unit of energy. One petajoule equals 947.817 billion Btu, 277.7778 gigawatt hours, or about 9.48 megatherms.
petameter (Pm)
a metric unit of distance equal to 1012 kilometers. This is equivalent to about 621.371 billion miles or 0.1057 light year. The distance from the earth to the nearest star (other than the sun) is about 40 petameters.
pF [1]
symbol for the picofarad.
pF [2]
a unit formerly used in agricultural science to measure "soil suction" or soil moisture tension. Soil moisture tension is the pressure that must be applied to the moisture in the soil to bring it to hydraulic equilibrium with an external pool of water. This was measured in pF units as the logarithm of the pressure in centimeters of water. Currently measurements are usually made directly in kilopascals (kPa).
pferdestärke (PS)
German word for horsepower, meaning the metric horsepower. The symbol ps is used for horsepower in both the Japanese and German automotive industies.
a traditional unit of liquid volume for beer in Austria. A Pfiff (the German word means "whistle") is a small quantity of beer. Traditionally it was equal to 1/2 Seidel, which would be about 177 milliliters (5.99 U.S. fluid ounces), but in current use it is generally 200 milliliters (6.76 U.S. fluid ounces), which is 2/3 of the metric Seidel in Austria.
symbol for the particle flux unit (see above).
pfund (pfd)
a traditional German weight unit corresponding to the English pound (see below). The pfund is equal to 16 unze or 32 lot. Traditionally the pfund varied in size from market to market, and the various German states adopted different standards, ranging from something close to the English pound (454 grams) to the Viennese pfund at about 1.2 pounds (560 grams). When Germany was unified in the late nineteenth century, the pfund was redefined as a metric unit equal to exactly 500 grams (about 1.102 31 pound). There's no change in the plural.
a logarithmic measure used to state the acidity or alkalinity of a chemical solution. The properties of a liquid solution we call "acid" are caused by the presence of hydrogen ions (H+). The pH of a solution is a measure of the concentration of these hydrogen ions. Technically, the pH of a solution is defined to be the negative logarithm of the concentration, measured in moles per liter. This unit is inverted in the sense that lower pH readings correspond to greater acidity, and therefore more hydrogen ions. Lowering the pH by 1.0 means multiplying the ion concentration by a factor of 10. Mathematically the scale is open at both ends, but in practice pH values usually fall in the range from 0 to 14. Pure water at 25 °C (77 °F) has a pH close to 7.0. Numbers below 7 indicate increasing acidity, while numbers above 7 indicate increasing alkalinity. The pH ("potential of Hydrogen") scale was invented in 1909 by the Danish chemist Søren Peter Lauritz Sørensen (1868-1939).
Ph Eur unit
a unit used in the European Union to measure the potency of a vitamin or drug, that is, its expected biological effects. For each substance to which this unit applies, the European Directorate for the Quality of Medicines has determined the biological effect associated with a dose of 1 Ph Eur unit. Other quantities of the substance can then be expressed in terms of this standard unit. In many cases, the Ph Eur unit is equal to the international unit (IU).
phi unit
a logarithmic unit used to measure grain sizes for sand, grit, and gravel. The 0 point of the scale is a grain size of 1 millimeter, and each increase of 1 in the phi number corresponds to a decrease in grain size by a factor of 1/2. Thus 1 phi unit is a grain size of 0.5 mm, 2 phi units is 0.25 mm, and so on; in the other direction, -1 phi unit corresponds to a grain size of 2 mm and -2 phi units to a size of 4 mm.
a logarithmic measure of sound loudness closely related to the decibel. Decibels are used for objective measurements, that is, they measure the actual pressure of the sound waves as recorded using a microphone. Phons are used for subjective measurements, that is, measurements made using the ears of a human listener. A sound has loudness p phons if it seems to the listener to be equal in loudness to the sound of a pure tone of frequency 1 kilohertz and strength p decibels. A measurement in phons will be similar to a measurement in decibels, but not identical, since the perceived loudness of a sound depends on the distribution of frequencies in the sound as well as the pressure of the sound waves. In the U.S., sound loudness is frequently measured in sones rather than phons: a sound of loudness s sones has loudness 10 log2 s + 40 phons. In English the unit is pronounced "fon" rather than "phone."
phot (ph)
the CGS unit of illuminance or illumination, equal to one lumen per square centimeter or 10 000 lux. See also lambert. The name is usually pronounced to rhyme with "not" rather than "note."
photon [1]
a quantum of light energy, or, more generally, of any form of electromagnetic energy having a single wavelength, direction, and polarization.
photon [2]
a former name for the unit of retinal illuminance now called the troland.
pi (π)
a very famous mathematical unit. The circumference of a circle is equal to pi multiplied by the diameter, so pi is equal to the ratio between the circumference and diameter of a circle, any circle. It turns out that pi is an irrational number, which means that its decimal expansion is nonterminating and nonrepeating. To 25 significant digits, pi equals 3.141 592 653 589 793 238 462 643. The Swiss mathematician Leonhard Euler (1707-1783) adopted the lower case Greek letter π (pi) for this ratio in 1737; although he was not the first to use it, he popularized it through his many mathematical writings. Pi sometimes appears to be a unit of angle measure equal to π radians or 180°. This is because mathematicians regard angle measurement as dimensionless (length divided by length) and therefore omit the unit "radians".
see pik, below.
pica (pi or pc)
a unit of length used by typographers and printers. One pica equals 12 points, 4.22 mm, or a bit less than 1/6 inch. "Pica type" is type that sets six lines to the inch. The origin of this word is not known. In the Catholic Church, a "pica" is a book of daily services, so it is possible that pica type got its name from being used to print picas. The unit is pronounced "pike-ah."
pico- (p-)
a metric prefix denoting 10-12 (one trillionth). The prefix, pronounced "peek-o" in English, is coined from the Italian piccolo, small.
picocurie (pCi)
a common unit of radioactivity, used to measure radioactivity occurring naturally in the environment. One picocurie equals 10-12 curie or 0.037 becquerel; this corresponds to one atomic disintegration about every 27 seconds--a very low rate of activity.
picofarad (pF)
a common unit of electric capacitance equal to 10-12 farad. This unit was formerly called the micromicrofarad (µµF).
picogram (pg)
a metric unit of mass equal to 10-12 gram or one millionth of a microgram.
picoliter (pl or pL)
a metric unit of volume equal to 10-12 liter or 1000 cubic micrometers. Engineers at Eastman Kodak recently reported a technique for producing ink droplets as small as several picoliters.
a unit of volume for champagne, equal to 1/4 bottle (187.5 milliliters).
picomole (pmol)
a unit of amount of substance equal to 10-12 mole. This unit is common in biochemistry; since a mole of a large organic molecule can be quite a large amount, a picomole is larger than one might think.
picosecond (ps)
a unit of time equal to 10-12 second.
picotesla (pT)
a unit of magnetic field strength equal to 10-12 tesla or 10-8 gauss.
a unit of weight comparable to the European quintal, widely used in East Asia during the colonial period. The picul is equal to 100 catties, typically about 133.3 pounds or 60.5 kilograms. In recent years the picul has been used as a metric unit equal to 60 kilograms (132.28 pounds) in Thailand or 50 kilograms (110.23 pounds) in China. The unit is pronounced "pickle."
the traditional foot of Spain. The pie equals 1/3 vara or 12 pulgadas (see below). The pie used in Spain is about 27.86 centimeters or 10.97 inches, but in Spanish Latin America the pie is generally longer. The Argentine length is 28.89 centimeters or 11.37 inches; in Texas, 1/3 vara is 11 1/9 inches or 28.22 centimeters.
piece (pc)
a unit of quantity, equal to 1. This unit, like count (ct) is used to indicate that a measurement represents an exact count of items.
the traditional French foot. Pieds of various lengths were used in France, but the one best remembered now is the royal foot (pied de roi), called the Paris foot in English and the foot (French measure) in Canadian law. The pied de roi equals about 32.48 centimeters or 12.79 inches; the official Canadian definition is 12.789 inches (32.484 06 centimeters). Today the word pied is sometimes used informally in France as a metric unit equal to 30 centimeters. In French Canada, pied is generally used to refer to the English foot.
the traditional foot of Italy. The unit, no longer used, varied considerably from one region to another; one common length was about 29.8 centimeters, but lengths of 34.8 cm and 38 cm were traditional in Venice and Bologna, respectively.
pieze (pz)
a metric unit of pressure, part of the "metre-tonne-second" system sometimes used by European engineers. The pieze is a pressure of one sthene per square meter, or 1000 newtons per square meter, or one kilopascal. This is not such a large pressure; one pieze equals 10 millibars or about 0.145 pound per square inch. The name of the unit comes from the Greek piezein, to press. The unit, spelled pièze in French, is pronounced "pee-ezz" in English.
pik or pic
a traditional unit of distance in the Eastern Mediterranean and Near East. The pik varied considerably but a typical value is about 28 inches (71 centimeters). This is an "arm" unit, like the Italian braccio and the Russian arshin.
a traditional British unit of volume, used for beer. A pin is very different from a pint: it is equal to 1/8 barrel or 4.5 Imperial gallons (20.457 liters). There are 2 pins in a firkin. See also polypin (below).
an informal unit of volume used in food recipes. Historically a pinch was defined as "an amount that can be taken between the thumb and forefinger" but without any definite equivalent in other units. Recently kitchen supply stores in the U.S. and other countries have begun selling sets of "minispoons" in which the spoon labeled "pinch" is designed to hold exactly 1/2 dash or 1/16 teaspoon, which is roughly 0.01 fluid ounce or 0.3 milliliter.
a traditional unit of area in Taiwan, equal to about 3.305 square meters (3.953 square yards). This is the same unit known in Korea as the pyong (see below).
pint (pt) [1]
a traditional unit of volume equal to 1/2 quart. There are three different quarts in use in Britain and the United States, and hence there are three different pints: [i] the U. S. liquid pint, equal to exactly 28.875 cubic inches, 16 fluid ounces, or approximately 473.176 milliliters; [ii] the U. S. dry pint, equal to 33.600 cubic inches or approximately 550.611 milliliters; and [iii] the British Imperial pint, equal to 20 British fluid ounces, 34.678 cubic inches or approximately 568.261 milliliters. The origin of the word pint is unclear. It may come from the Latin pincta, painted, referring to a marking at the one-pint level on a larger container.
pint (pt) [2]
a traditional unit of volume in Scotland equal to 2 choppins or 4 mutchkins. The Scots pint varied with time and locality, but it was eventually standardized as the volume of the Stirling jug, a vessel holding about 104.2 cubic inches or 1.708 liters. This is almost exactly 3 Imperial pints or 3.6 U.S. liquid pints.
pint (pt) [3]
a unit of volume used in South Australian pubs. A pint of beer is generally 425 milliliters in South Australia, or roughly 3/4 Imperial pint (15 fluid ounces). In Queensland a pint glass typically holds 560 milliliters, very nearly an Imperial pint.
pint (pt) [4]
a British unit of volume, known as the reputed pint, equal to 2/3 of the standard Imperial pint. This is exactly 13/3 fluid ounces, 23.12 cubic inches, or 378.841 milliliters.
the smallest measured change in a currency conversion rate. This depends on the relative values of the two currency units: in converting euros to U.S. dollars, for example, a pip is 0.0001, but in converting U.S. dollars to Japanese yen a pip is 0.01. This unit is also called a tick [2].
a traditional Portuguese unit of liquid volume, originally very similar in size to the English pipe (see next entry). The pipa has become a metric unit equal to exactly 500 liters, which is 0.5 cubic meter, 132.085 U.S. gallons, or 109.996 British Imperial gallons.
like the butt, the pipe is a traditional unit of liquid volume generally equal to 2 hogsheads. In the U.S., this means a pipe equals 126 U.S.gallons, about 16.844 cubic feet or 476.96 liters. In Britain it's more complicated, because traditional British hogsheads were of different sizes depending on what they contained. The British pipe was usually used as a wine measure, but even different types of wine had different size pipes.
another name for "characters per inch," a unit used in printing.
a picture element. Pixels do not have a fixed size; their diameters are generally measured in micrometers (microns). Although the pixel is not a unit of measurement itself, pixels are often used to measure the resolution (or sharpness) of images. As a hypothetical example, a 600 x 1000 pixel image has 4 times the pixel density and is thus 4 times sharper than a 300 x 500 pixel image, assuming the two images have the same physical size.
usually a symbol for "pack." In Dutch, the symbol for paardekracht (the metric horsepower).
an MKS unit of "action" (energy expended over time) or of angular momentum. The planck is equal to 1 joule second (J·s) or about 0.7375 foot pound second (ft·lb·s). The unit honors the German physicist Max Planck (1858-1947), the originator of quantum theory.
Planck length
a unit of distance representing the scale at which gravity, and perhaps space itself, becomes quantized (discrete) rather than continuous. This is the shortest distance that is meaningful in our understanding of the laws of physics. The Planck length is defined to be the square root of Gh/c3, where G is the universal gravitational constant, h is Planck's constant, and c is the speed of light. This makes the Planck length about 4.051 x 10-35 meter.
Planck time
a unit of time equal to the time required for a photon moving at the speed of light to travel the distance of one Planck length. This is the shortest time that is meaningful in our understanding of the laws of physics, representing the scale at which time itself may become quantized (discrete) rather than continuous. The Planck time is about 1.351 x 10-43 second.
Platonic year
a unit of time used in astronomy. The earth's axis of rotation is not fixed in space; the attraction of the moon causes it to slowly trace out a circle in the sky. This motion, called precession, changes the orientation of the sky as seen from the earth's surface: the poles appear to shift their locations and the sun's point of crossing the equator slowly rotates through the constellations of the Zodiac. The Platonic year is the length of time required for one complete precessional rotation: about 25 800 years. The unit is named for the ancient Greek philosopher Plato (ca. 428-348 BCE). It is sometimes called the great year.
a suffix used to create units of quantity. Thus triplet, for example, is a unit of 3. However, words having this ending are also used to mean one member of the group rather than the group as a whole.
an ancient Greek unit of distance equal to 100 Greek feet or 1/6 stadion. The Greek foot was slightly longer than the English foot, so the plethron was approximately 100-105 English feet or 31-33 meters. Often represented as the length of a cord, the unit was frequently used for measuring land areas. The plural is plethra.
a suffix used to create large numbers. The number n-plex is 10n, which is 1 followed by n zeros. Thus a googol is 100-plex, for example. The American mathematician Edward Kasner, who invented the googol in 1938, also defined a "googolplex" to be 1 followed by a googol of zeros, thus suggesting this generalization. See also dex and -minex.
plf [1]
symbol for pounds per linear foot (lbm/ft), a common unit of load in engineering. 1 plf = 1.488 164 kilograms per meter (kg/m). The symbol is also used sometimes for pounds of force per linear foot (lbf/ft), in which case 1 plf = 14.593 90 newtons per meter (N/m).
plf [2]
a common abbreviation for "per linear foot." In this use, 1 plf = 3.280 840 per meter (/m).
a common abbreviation for pounds per linear inch. For load, 1 pli = 17.857 97 kg/m. For pounds of force per linear inch, 1 pli = 175.1268 N/m.
plotter unit
a unit of distance used in typography equal to 1/40 millimeter or 25 micrometers. This, the smallest distance addressed by Hewlett Packard plotters, has become a fairly familiar term in digital graphics design.
an ending used to indicate the number of folds or layers in an object; thus 4-ply means having 4 layers. The suffix comes from the Latin plicare, to fold.
PM or pm
abbreviation for the Latin post meridiem, "after noon," used after a time to indicate that the time occurs after 12:00 noon. Thus 4:30 pm is the same as 16:30. The notations "AM" and "PM" are used extensively in English speaking countries and especially in the United States, where time is usually not stated on a 24-hour basis. Note: By convention, midnight is represented as 12:00 am and noon as 12:00 pm. In text, however, it is best to avoid the use of 12:00 am or 12:00 pm since the reader may not be aware of these conventions.
abbreviation for "peak music power output," which is often claimed by electronics manufacturers as a unit measuring the effective power output of amplifiers, stereo systems, etc. Buyer beware! There is no industry standard for this "unit," so it is impossible to determine just what it means.
a symbol for "nominal pressure," a measure used for rating piping, valves, fittings, etc. Nominal pressure is essentially the pressure rating of the piping system, measured in bars at a temperature of 20 °C (68 °F). (One bar equals 100 kilopascals or approximately 14.5038 pounds per square inch in traditional English units.) Industrial standards organizations, such as the American National Standards Institute (ANSI), set standards for pipes and fittings based on PN ratings; these standards specify in detail the size, composition, and strength of each component.
an abbreviation for preferred noise criterion, a unit used in engineering to measure the level of background noise in rooms or other enclosed spaces. Introduced in 1971, the unit is similar to the older noise criterion (NC), but a PNC rating requires lower levels of high and low frequencies than the corresponding NC rating. PNC ratings below 40 are generally required for residential or classroom spaces. PNC ratings are typically 10-15% lower than raw measurements of the sound level in decibels.
abbreviation for protein nitrogen unit, a measure of the potency of the compounds used by doctors in allergy skin tests. One PNU is defined as 0.01 microgram (µg) of phosphotungstic acid-precipitable protein nitrogen. Unfortunately, the potency measurements depend on the technique of measurement used, so results of one manufacturer are not comparable to those of another manufacturer. As a result, although PNU's are still used, they are being replaced by bioequivalent allergy units (BAU), which are measured by actual skin testing using reference preparations of standard potency.
point (pt) [1]
a unit of angle measure equal to 1/32 of a circle. Compasses are read by suspending the compass needle over a compass card traditionally inscribed with a 32-point star. Each point of the star represents a named direction; for example, the first five points from north towards east are labeled North, North-by-East, North-Northeast, Northeast-by-North, and Northeast. The difference between two directions can be expressed as a certain number of these compass points. One point equals 11°15' of arc or pi/16 radians.
point (pt) [2]
a unit of length used by typographers and printers. When printing was done from hand-set metal type, one point represented the smallest element of type that could be handled, roughly 1/64 inch. Eventually, the point was standardized in Britain and America as exactly 1/72.27 = 0.013 837 inch, which is about 0.35 mm (351.46 micrometers). In continental Europe, typographers traditionally used a slightly larger point of 0.014 83 inch (about 1/72 pouce, 0.377 mm, or roughly 1/67 English inch), called a Didot point after the French typographer Firmin Didot (1764-1836). In the U.S., Adobe software defines the point to be exactly 1/72 inch (0.013 888 9 inch or 0.352 777 8 millimeters), a unit sometimes called the big point (bp). The German standards agency DIN has proposed that all these units be replaced by multiples of 0.25 millimeters (1/101.6 inch). See also kyu.
point (pt) [3]
a percentage point (see above).
point (pt) [4]
a unit of mass used for precious stones such as diamonds. One point equals 0.01 carat, or exactly 2 milligrams.
point (pt) [5]
a unit of quantity equal to 1. This unit is used to express changes in an arbitrary score or index, such as the score in an athletic contest. In finance, a change of one point in the Dow Jones average or similar indices represents a change of 1.00 in the index.
point (pt) [6]
a unit used to represent the smallest significant change in an arbitrary ratio. This usage is common in sports. Most sports "averages" are actually ratios of successful performances divided by attempted performances; baseball's batting average is a good example. These ratios are computed to a fixed number of decimal places, usually three, and a point represents a change of 1 in the last decimal place. Thus the batting averages .314 and .302 are said to differ by 12 points.
point (pt) [7]
another name for a mil [1], a unit of distance equal to 0.001 inch. Points are used with this meaning to measure the thickness, or caliper, of paper or card stock in the paper industry. One point equals 25.4 micrometers or microns.
point (pt) [8]
a measure of the specific gravity of a liquid, typically used in brewing and winemaking. Specific gravity is the mass of a sample of the liquid divided by the mass of an equal volume of pure water. It is a dimensionless (unit-less) number, typically a little larger than 1. Each "point" represents an increase of 0.001 above 1. For example, a liquid of specific gravity 1.048 is described as 48 point.
point [9]
a medieval unit of time equal to 1/5 hour, or 12 minutes. The point was divided into 8 moments.
poise (P, Ps, or Po)
a CGS unit of dynamic viscosity. Viscosity is a frictional property (actually, several related properties) of liquids and gases: due to friction between molecules, the liquid or gas resists flowing to a greater or lesser extent. Dynamic viscosity is measured by stating the force needed to move a standard area of one layer of the liquid or gas with respect to another layer, the two layers being parallel and separated by a standard distance filled with the same liquid or gas. If a force of one dyne is needed to move one square centimeter of the liquid or gas relative to a second layer one centimeter away at a speed of one centimeter per second, then the viscosity is one poise. The unit is named for the French physician Jean Louis Marie Poiseuille (1799-1869). Generally speaking, liquids have viscosities measured in centipoises and gases have viscosities measured in micropoises. The SI recognizes no named unit for dynamic viscosity; in SI units, one poise equals 0.1 pascal second (Pa·s). The poise is also equivalent to 14.5038 x 10-6 reyn. P is the proper symbol for the unit, but Ps and Po are also used.
poiseuille (Pl)
an MKS unit of dynamic viscosity equal to 1 pascal second or 10 poises or 1000 centipoises (cP). The poiseuille has been proposed, but not accepted, as an SI derived unit. See previous entry for more detail.
an empirical unit of indoor air pollution introduced by the Danish environmental scientist P.O. Fanger in 1988. One olf is defined as the air pollution produced by one "standard person", and one decipol is the perceived air pollution level in a space having a pollution source of strength one olf and ventilation with unpolluted air at the rate of 1 liter/second. In practice, nearly all measurements are made in decipols.
pole [1]
another name for a rod [1].
pole [2]
a unit of area equal to one square pole [1]. A pole of area covers exactly 272.25 square feet or about 25.292 85 square meters. There are 40 poles in a rood and 160 poles in an acre.
pole [3]
see unit magnetic pole.
the Portuguese "inch" unit, equal to 1/12 pé, 2.777 centimeters, or 1.093 inches.
pollex (poll)
the Latin word for "thumb," sometimes used to mean the inch in botanical descriptions.
an informal unit of volume for beer and other alcoholic beverages, used mostly in Britain. A polypin of beer comes in a plastic container, often inside a rectangular cardboard box; it holds 32-36 Imperial pints (18.2-20.5 liters) or, in the metric version, exactly 20 liters (5.28 U.S. gallons). The word polypin is actually a registered trademark of Biovision GmbH; it is the name of the polythene plastic used for the lining of the container. The traditional British pin of beer (see above) is exactly 36 pints.
abbreviation for pump octane number. See octane number.
poncelet (p)
a unit of power formerly used in France but now obsolete. The poncelet is defined to be the power required to raise a mass of 100 kilograms at a velocity of 1 meter per second. This is equivalent to 980.665 watts or exactly 4/3 metric horsepower (1.315 traditional horsepower). The unit was named for the French mathematician and engineer Jean-Victor Poncelet (1788-1867). It was replaced in French engineering by the metric horsepower (cheval vapeur).
pond [1]
the Dutch pound, historically about 494 grams (1.089 English pounds). This unit was also used in the former Dutch Indies (now Indonesia) and throughout Southeast Asia. In the Netherlands, the pond has been reinterpreted now as a metric unit equal to exactly 500 grams (1.1023 pounds), like the German pfund.
pond (p) [2]
a metric unit of force, formerly more common and still used for some purposes. A pond is equal to a gram weight, that is, the gravitational force on a mass of one gram; thus it is equal to 980.665 dynes or 0.002 204 622 6 pounds of force. The kilopond was used more often than the pond. The name of the unit is from the Latin pondus, weight.
pony [1]
a small glass for liquor. In the U.S., a pony generally holds exactly 1 (U.S.) fluid ounce or about 29.6 milliliters. The word "pony" means a small horse, hence anything smaller than normal--in this case, a smaller shot glass.
pony [2]
a small glass for beer. In New South Wales, Victoria, South Australia, and Western Australia, a pony of beer holds 140 milliliters (about 5 British fluid ounces).
pony [3]
a small keg of beer. In the U.S., a pony keg holds 1/4 barrel or 7.75 U.S. gallons (about 29.34 liters).
see pud.
in the paper industry, "porosity" is generally measured by the time (in seconds) required for 100 cubic centimeters of air to pass through one square inch of the paper at a standard pressure difference. Thus the porosity unit is square inch seconds per deciliter (s·in2/dl). This is sometimes called a Gurley unit after the name of a test procedure. In SI units, 1 s·in2/dl = 6.4516 seconds per meter column of air (s/m).
pot [1]
a traditional unit of volume in many countries of Europe, roughly comparable to the liter or to the English quart. In Switzerland, the pot is now a metric unit equal to 1.5 liters. In Belgium, the pot is interpreted as 1.5 liters for dry quantities, but only 0.5 liters for liquids. The traditional pot is equal to 0.967 liters in Denmark and to 0.965 liters in Norway.
pot [2]
a unit of volume used in Australian pubs. A pot of beer is 285 milliliters in Queensland and Victoria, 575 milliliters in Western Australia.
pot [3]
a traditional unit of volume in Jersey (Channel Islands). Still used to measure milk, the pot has always been roughly comparable to 1/2 gallon. In the current definition, one pot equals 69.5 Imperial fluid ounces (1.7375 quarts) or about 1.975 liters.
a traditional unit of volume equal to 1/2 gallon. The unit's name is from the French potel, a type of bottle.
the French "inch" unit, equal to 1/12 pied (see above). Based on the pied de roi, the pouce equals about 1.066 inches or 2.707 centimeters. The word pouce means "thumb" in French.
an acronym for one pound per million yards, a unit of yarn density formerly used in the U.S. textile industry. One poumar equals about 0.496 055 tex.
pound (lb, lbm, or #) [1]
a traditional unit of mass or weight. The Romans used a pound (the libra pondo, "pound of weight") divided into 12 ounces. All the countries of western Europe used similar units, divided into 12 or 16 ounces, until the advent of the metric system. 12-ounce pounds were common in Italy and southern France, but in Spain and northern Europe 16-ounce pounds became the norm. The word libra is used for this unit in Italy, Spain, and Portugal; in France it is called the livre. Further north, the Latin word pondo ("weight") is the origin of the names of the English pound, Dutch pond, Danish pund, German pfund, and Russian funt. In England, two different "pound" units became standard. The unit now in general use in the United States is the avoirdupois pound, so-called from a French phrase avoir du poids, literally "goods of weight," indicating simply that the goods were being sold by weight rather than by volume or by the piece. The avoirdupois pound is divided into 16 ounces. By international agreement, one avoirdupois pound is equal to exactly 453.592 37 grams; this is exactly 175/144 = 1.215 28 troy pounds. See avoirdupois weights for additional information. The traditional symbol lb stands for libra, the Latin word for the unit. The avoirdupois pound is sometimes abbreviated lb av or lb ap to distinguish it from the less common troy pound. The symbol lbm is used in science to distinguish the pound of mass from the pound of force (lbf): see pound force, below.
pound (lb t or lb or #) [2]
a second traditional unit of mass or weight. The troy pound, named for the French market town of Troyes, was the unit used in England by apothecaries and jewelers. The troy pound is divided into 12 ounces like the Roman pound. One troy pound is 373.242 grams, or exactly 144/175 = 0.822 858 avoirdupois pounds (13.165 72 avoirdupois ounces). The troy and avoirdupois pounds are connected by the grain: there are 5760 grains in a troy pound and 7000 grains in an avoirdupois pound. See troy weights for additional information. The troy pound should be abbreviated lb. t. to distinguish it from the more common avoirdupois pound.
pound (lbf or lb) [3]
a traditional unit of force; see pound force, below.
pound (lb) [4]
a traditional unit measuring the weight of paper; see pound weight, below.
poundal (pdl or pl)
an English unit of force used in engineering. Since traditional measuring systems, including the English system, did not distinguish between force and mass units, the poundal was defined to provide a unit clearly measuring force rather than mass. One poundal is the force that accelerates a mass of one pound at the rate of one foot per second per second. Since the acceleration of gravity averages about 32.174 ft/sec2 at the Earth's surface, one poundal is about 1/32.174 = 0.031 081 pound of force. One poundal is also equal to approximately 0.138 255 newton, or 13 825.5 dynes. The newton, the SI unit, is now the preferred unit of force in engineering and technical work. The poundal was invented in the 1870s by the British mathematician James Thomson, who also named the radian.
pound cut (lb cut)
a traditional unit of concentration for shellac in the U.S. One pound cut means that the shellac was manufactured by dissolving one pound of dry, bleached shellac in one gallon of alcohol solvent (about 120 grams of shellac per liter of solvent). The most common concentrations sold are 3, 4, and 5 lb cut, but diluted solutions of 1/4 to 1 lb cut are sometimes used as sealers or polishes.
pound foot (lbf ft or lb ft)
a traditional unit of torque. Torque is the tendency of a force to cause a rotation; it is the product of the force and the distance from the center of rotation to the point where the force is applied. Thus it can be measured in pounds of force times feet of distance. One pound foot is equal to approximately 1.355 818 newton meter (N·m) in SI units. Algebraically, torque has the same units as work or energy, but it is a different physical concept. To stress the difference, scientists and engineers traditionally measure torque in pound feet (or newton meters) and work or energy in foot pounds (or joules).
pound force (lbf or lb)
a traditional unit of force. Traditional measuring systems did not distinguish between force and mass units. A force of one pound is simply the gravitational force experienced at the Earth's surface by a mass of one pound. To compute this force, we multiply the mass by the acceleration of gravity, following Newton's law F = ma. Since one pound of mass is 0.453 592 kilograms and the acceleration of gravity averages 9.806 65 meters per second per second at the surface of the Earth, one pound force equals the product of these two numbers, 4.448 221 615 newtons. The symbol lbf should be used for the pound force to distinguish it from the pound of mass.
pound mass (lbm)
see pound [1], above.
pound mole (lbmol)
a unit of amount of substance. One pound mole of a chemical compound is the same number of pounds as the molecular weight of a molecule of that compound measured in atomic mass units. Thus the pound mole is equal to exactly 453.592 37 moles.
pound per square foot (lbf/ft2 or psf)
a traditional unit of pressure. 1 psf equals about 47.880 pascals (Pa), 0.478 80 millibars (mb), or 0.192 79 inch of water (in WC).
pound per square inch (lbf/in2 or psi)
a traditional unit of pressure. 1 psi equals 144 pounds per square foot (psf), 6.894 75 kilopascals (kPa), 68.9475 millibars (mb), 2.036 inches of mercury (in Hg), 27.7612 inches of water (in WC), or 70.5134 centimeters of water (cm H2O). See below for related notations such as "psig."
pound weight (lb wt or lb)
a traditional U.S. unit measuring the weight or thickness of paper. Paper is described as, say, 24 pound weight if one ream (500 sheets) cut in a standard size (called the base size or basis size) has a mass of 24 pounds. For bond paper, the base size is 17 inches by 22 inches (43.18 by 55.88 centimeters), exactly four times the area of an 8.5 inch by 11 inch sheet. This means a ream of 8.5 inch by 11 inch, 24-pound bond paper has a mass of 6 pounds. A table of basic sizes is provided. The metric measure of paper weight is the areal density in grams per square meter (g/m2 or "gsm"). 1 lb wt is equivalent to 3.76 g/m2 for bond paper, 1.48 g/m2 for text stock, 2.70 g/m2 for card stock, and varying amounts for the other types of paper.
the ancient Greek foot, a unit of distance equal to about 30.7 centimeters, a little longer than the modern English foot. The plural is podes. The pous was divided into 16 daktylos (digits). There were 100 podes in a plethron and 600 in a stadion.
power (x) [1]
a unit expressing the magnifying power of an optical system. The power is defined to be the angular diameter of the image formed by the system divided by the angular diameter of the original object being observed. In simple telescopes this is equal to the focal length of the primary objective (the big lens or mirror) divided by the focal length of the eyepiece lens. For binoculars, the power is customarily followed by the diameter of the objective lenses, in millimeters, so "8x40" indicates binoculars with a magnifying power of 8 and lenses of diameter 40 mm.
power (x) [2]
a measure of the focal power of a lens equal to 40 times the focal length, or 40 divided by the refractive power in diopters. For example, a 2.00 diopter lens in a pair of reading glasses is also described as 20 power.
power (x) [3]
a term indicating that a measurement is a multiple of some standard quantity. For example, in computer technology, a 16x CD-ROM drive spins a disk 16 times faster than a "standard" speed drive.
a traditional abbreviation for "pages."
ppb, ppm, ppq, ppt, pptr
abbreviations for units of proportion: ppb = part per billion (10-9), ppm = part per million (10-6), ppq = part per quadrillion (10-15), and ppt = part per trillion (10-12), respectively. However, the abbreviation "ppt" is also used sometimes for part per thousand (10-3). To avoid this confusion, "pptr" is an alternate abbreviation for part per trillion.
ppcm, ppi [1]
abbreviations for pixels per centimeter and pixels per inch, respectively. A pixel is a single "picture element", so these units measure the resolution, or fineness, of an image.
ppi [2]
abbreviation for pages per inch, a measure of paper thickness.
ppi [3]
abbreviation for pores per inch, a measure of porosity for polyurethane foams and other industrial foam and filter products.
printer's ream
a unit of quantity for paper. An ordinary ream is 480 or 500 sheets; a printer's ream is 516 sheets. The additional amount is to allow for sheets that may be spoiled in shipment. The unit is also called the perfect ream.
prism diopter (PD)
a unit used in optics to measure the deflection of light by a prism. One prism diopter represents a deflection of 1 centimeter measured at a distance of 1 meter from the prism. Mathematically, the deflection in prism diopters is equal to 100 times the tangent of the angle through which the path of the light is bent in passing through the prism.
proof (prf)
a traditional unit of proportion used for measuring the strength of distilled liquors, including medicinal solutions of alcohol as well as alcoholic beverages such as whiskey. The proof rating of a liquor is the alcohol content of the liquid expressed as a percentage of the alcohol content of a standard mixture, called the proof liquor. In the United States, the proof liquor is legally defined so to contain exactly 50% alcohol measured by volume. As a result, the U. S. proof rating is equal to exactly twice the percentage of alcohol present, measured by volume. Thus "86 proof" means 43% alcohol. In Britain proof ratings are no longer used, but the former proof liquor contained 57.27% alcohol by volume. This means that 86 proof Scotch, in the U.S., was formerly 75 proof in Britain.
abbreviation for the peripheral resistance unit, used in physiology and medicine to assess blood flow in the capillaries. A measurement in PRU's is equal to the blood pressure in millimeters of mercury divided by the flow rate in milliliters per minute. That is, 1 PRU equals 1 mmHg·min/mL = 133.3 Pa·min/mL (or, in proper SI units, almost exactly 8 GPa·s/m3).
see pferdestärke above.
psi, psia, psid, psig
traditional symbols for pressure units used in hydraulics and plumbing. psi is a symbol for pound per square inch (see above). psig is a symbol for pound per square inch gauge; this means that the pressure has been read from a gauge which actually measures the difference between the pressure of the fluid and the pressure of the atmosphere. psia means pound per square inch absolute, which is the total pressure including the pressure of the atmosphere. psid, pound per square inch differential, is a symbol for a difference between two pressures, neither of which is atmospheric pressure. Corresponding symbols for pound per square foot (psf, etc.) are also used.
PSU or psu
an abbreviation for practical salinity unit, a standard measure of the salinity of seawater. The "unit" is actually a dimensionless (unitless) ratio obtained by measuring the conductivity of the water sample. Seawater of salinity 35 PSU has the same conductivity as a standard solution of potassium chloride (KCl) with a concentration of 3.243 56 % by mass; a sample of salinity 1 PSU would have conductivity 1/35 that of the standard solution. With this definition, measurements in PSU are very nearly the same as direct measurements of salt ion concentration in parts per thousand.
A unit of distance used during the colonial era in China. The pu equals 5 ch'ih, 5.875 feet, or 1.7907 meters.
pud or pood
a traditional unit of weight in Russia. The pud equals 40 funte or 1/30 packen; this is about 16.381 kilograms or 36.11 pounds. The plural is pudi.
puff (pF)
an informal name for the picofarad , a unit of electric capacitance.
the traditional Spanish inch, equal to 1/12 pie (see above). The pulgada varies from about 23.2 to 24.1 millimeters (0.913 to 0.949 inch).
a measure of the angular deflection in an overhead utility line at a pole where the line changes direction. In the U.S., the measurement is defined by drawing an imaginary line between two points on the utility line 100 feet from the corner pole, one point in each direction. The pull is then defined to be the minimum distance between this imaginary line and the corner pole (in feet). Pull p is related to the angle a of deflection by the formula p = 100·sin(a/2); this quantity is directly proportional to the sideways force exerted on the corner pole.
a traditional unit of liquid volume. The puncheon is often reckoned as equal to 70 gallons. In the U. S. system that would be about 9.358 cubic feet or 264.98 liters; in the British Imperial system it would be about 11.238 cubic feet or 308.34 liters. There are other versions of the unit; in one version a puncheon of wine equals 84 wine (or U.S.) gallons (roughly 308 liters); in another, a puncheon of beer equals 72 gallons (roughly 272.5 liters).
the Scandinavian pound, now reinterpreted as a metric unit equal to 500 grams (1.1023 pounds), like the German pfund. The traditional Swedish (Stockholm) pund was equal to about 425.1 grams (14.995 ounces).
a small square or sometimes rectangular container for fruit or vegetables, such as strawberries or bean sprouts. When used as a unit of measure, a punnet is generally the same thing as a dry pint in the U.S. or an Imperial pint in Britain; see pint [1] above. However, grocers use punnets of several sizes to package berries, fresh mushrooms, etc.
a traditional Korean unit of area equal to about 3.306 square meters or 3.954 square yards. The pyong is widely used in Korea to measure areas both inside and outside buildings. The Taiwanese ping (see above) corresponds closely to the pyong.
a unit used to measure the heat flow delivered by solar radiation. The pyron is equal to one calorie per square centimeter per minute, which is exactly 697.8 watts per square meter (W/m2) if the IT calorie is used in the definition, or 697.633 W/m2 if the 15° calorie is used. The name is coined from the Greek word pyr for fire.

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Checked and revised January 23, 2002