- 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 
- 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 
- 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 
- 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 
- 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
- palm 
- a name sometimes used in Dutch for the decimeter (10 centimeters, or about
- 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
- 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
- part 
- 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
- part 
- a medieval unit of time equal to 1/15 hour or 4 minutes.
- part 
- 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 =
- 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
- 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
- pearl grain
- a unit of mass equal to 1/4 carat or 50 milligrams;
see grain .
- 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  as used
with sums of money. An expression such as "the fourth penny" means 1/4 of
- pennyweight (dwt or pwt)
- a unit of weight in the traditional troy system (see also pound
), 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 
- a unit of quantity equal to 5.
- pentad 
- 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 
- an alternate name for the rod  (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
- perch 
- a unit of area equal to one square perch . 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 
- 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 
- 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  or perch 
- 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  or perch 
- a traditional unit of area in French North America and also in Mauritius, equal to one square
perche . 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
- 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 
- symbol for the picofarad.
- pF 
- 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
- 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 
- a quantum of light energy, or, more generally,
of any form of electromagnetic energy having a single wavelength, direction,
- photon 
- 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,
- 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
- 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
- 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 du roi), called
the Paris foot in English and the foot (French measure) in Canadian
law. Under a 1668 reform the pied du roi was fixed at 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. Note: the French language Wikipedia has a good article on the pied and its history.
- 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
- 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) 
- 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) 
- 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.
- pint (pt) 
- 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) 
- a British unit of volume, known as the reputed pint, equal to 2/3 of the standard Imperial pint. This is exactly 13-1/3 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
- 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
- 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
- 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
- plf 
- 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 
- 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,
- 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
- 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) 
- 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
- point (pt) 
- 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) 
- a percentage point (see above).
- point (pt) 
- a unit of mass used for precious stones such as diamonds. One point equals
0.01 carat, or exactly 2 milligrams.
- point (pt) 
- 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) 
- 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) 
- another name for a mil , 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) 
- 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 
- 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. There is also some inconsistency in the plural for the unit, with both "poise" and "poises" in use.
- 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 
- another name for a rod .
- pole 
- a unit of area equal to one square pole . 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 
- 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
- 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
- pond 
- 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
- pond (p) 
- 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
- pony 
- 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 
- 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
- pony 
- 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 
- 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 
- 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 
- 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 #) 
- 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 #) 
- 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) 
- a traditional unit of force; see pound force, below.
- pound (lb) 
- a traditional unit measuring the weight of paper; see pound weight,
- 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
- pound mass (lbm)
- see pound , 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
- 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) 
- 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) 
- 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) 
- 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
- ppcm, ppi 
- 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 
- abbreviation for pages per inch, a measure of paper thickness.
- ppi 
- 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
- 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
- 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  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