† sq ft, sq mi, ... are also used but ft^{2} is preferred
§ Although the use of the abbreviation psi is common, it is not recommended.
[Ω]  Character entity reference code for entering the symbol Ω in html text.
The html code for superscript 2 is ² e.g. "ft²" for ft²
* Spelling varies by country:
 metre (British, Australian, Canadian and New Zealand English;
French)
 meter (American English, Danish, Dutch, German, Hungarian, Norwegian,
Slovak, and Swedish)
 metr (Czech, Polish, Russian, Ukrainian)
 metras (Lithuanian)
 metri (Finnish)
 metro (Basque, Italian, Portuguese, Spanish)
Same for liter (American, ...) and litre (British, ...)
See:
Recommended Unit Symbols, SI Prefixes, and Abbreviations at IEEE
Units of Measurement at Russ Rowlett's page at the UNC
Abbreviations and Standard Abbreviations at the Naval NRaD Writing and Editorial Guidelines
Measurement prefixes:
Symbol Rules
 Don't use a period to indicate an abbreviation or s for plural.
e.g. 2 min, NOT 2 min. or 2 mins.
 Symbols are casesensitive and must be written as they are defined.
A for ampere NOT a and kW for kilowatt NOT KW
 The superscripts ^{2} and ^{3} are always used for "square"
and "cubic", respectively.
e.g. km^{2}, NOT sq km.
 A raised dot (also called a middle dot or halfhigh dot, "•" [•]) is recommended when symbols are multiplied. It is permissible to use a space instead, but symbols should not be placed next to one another with nothing between them.
e.g. A·h is the recommended symbol for the ampere hour. A h is also permitted, but NOT Ah
 The slash (solidus) / is used for "per" and only one /
e.g. m/s^{2} NOT m/s/s or meters per sec per sec.
 Symbols are separated from the numerical quantity they follow by a space.
 Prefixes appear next to the symbol without a space. e.g. MHz not M HzSymbols are separated
 from the numerical quantity they follow by a space.
* Thus 5 kilograms is written 5 kg, not 5kg.
See Also: Symbols at UNC
fractions 
number 
scientific notation 
name 
abbreviation 
0.1 
1E1 
deci 
d 
0.01 
1E2 
centi 
c 
0.001 
1E3 
milli 
m 
0.0001 
1E4 
decimilli 
dm 
0.0001 
1E5 
centimilli 
cm 
0.000,001 
1E6 
micro micron * 
µ 
0.000,000,001 
1E9 
nano 
n 
0.000,000,000,001 
1E12 
pico 
p 
0.000,000,000,000,001 
1E15 
femto 
f 
0.000,000,000,000,000,001 
1E18 
atto 
a 
There is also a special unit for length called
angström, which is 1E10 of a meter, the order of
magnitude of the size of an atom. It is not part of the SI standard but is still used.
* In 1968 the CGPM (Conférence Générale des Poids et Mesures) decided to drop the micron as an approved unit and recommend that micrometers be used instead. Microns, however, are still in common use.
multiples 
number 
scientific notation 
prefix 
abbreviation 
name 
10 
1E1 
deka 
D 

100 
1E2 
hekto 
h 
hundred 
1,000 
1E3 
kilo 
k 
thousand 
0.0001 
1E4 
myria 
ma 

1,000,000 
1E6 
mega 
M 
million 
1,000,000,000 
1E9 
giga 
G 
billion 
1,000,000,000,000 
1E12 
terra 
T 
trillion 
1,000,000,000,000,000 
1E15 
peta 
P 
quadrillion 
1,000,000,000,000,000,000 
1E18 
exa 
E 
quintillion 
1,000,000,000,000,000,000,000 
1E21 
zetta 
Z 
sextillion 
1,000,000,000,000,000,000,000,000 
1E24 
yotta 
Y 
septillion 
International System of Units (SI) or (BIPM, for Bureau International des Poids et Mesures) Base Units
Source: Russ Rowlett's Units of Measurement at the UNC.
 Distance
 meter (m)  Originally, the meter was designed to be one
tenmillionth of a quadrant, the
distance between the Equator and the North Pole. (The Earth is
difficult to measure, and a small error was made in correcting for
the flattening caused by the Earth's rotation. As a result, the
meter is too short by about 0.013%. That's not bad for a
measurement made in the 1790's.) For a long time, the meter was
precisely defined as the length of an actual object, a bar kept at
the International Bureau of Weights and Measures in Paris. In
recent years, however, the SI base units (with one exception) have
been redefined in abstract terms so they can be reproduced to any
desired level of accuracy in a wellequipped laboratory. The 17th
General Conference on Weights and Measures in 1983 defined the
meter as that distance that makes the speed of light in a vacuum
equal to exactly 299 792 458 meters per second. The speed of light
in a vacuum, c, is one of the
fundamental constants of nature. Since c defines the meter
now, experiments made to measure the speed of light are now
interpreted as measurements of the meter instead. The meter is
equal to approximately 1.093 613 3 yards,
3.280 840 feet, or 39.370 079
inches. Its name comes from the
Latin metrum and the Greek metron, both meaning
"measure." The unit is spelled meter in the U.S. and
metre in Britain; there are many other spellings in various
languages (see Spelling of Metric
Units).
See size for other units e.g. Lightyear, AU (Astronomical Unit), Parsec,
 Mass
 kilogram (kg)  The kilogram is defined as the mass of the standard kilogram, a platinumiridium bar in the custody of the International Bureau of Weights and Measures (BIPM) near Paris, France. Copies of this bar are kept by the standards agencies of all the major industrial nations, including the U.S. National Institute of Standards and Technology (NIST). One kilogram equals exactly 1000 grams, or about 2.204 622 6 pounds. By design, this is approximately the mass of a liter of water.
 Time
 second (sec) The name simply means that this unit is the second division of the hour, the minute being the first. The second was defined as 1/86 400 mean solar day until astronomers discovered that the mean solar day is actually not constant (see day). The definition was then changed to 1/86 400 of the mean solar day 1900 January 1. Since we can't go back and measure that day any more, this wasn't a real solution to the problem. In 1967, scientists agreed to define the second as that period of time which makes the frequency of a certain radiation emitted by atoms of cesium133 equal to 9 192 631 770 hertz (cycles per second). In other words, if we really want to measure a second, we count 9 192 631 770 cycles of this radiation. This definition allows scientists to reconstruct the second anywhere in the world with equal precision.
 Electric current
 ampere (A) named for the French physicist
AndréMarie Ampère (17751836), one of the pioneers
in studying electricity. The official definition of the ampere
goes like this: suppose we have two parallel conductors,
infinitely long and having negligible cross section. Place these
conductors one meter apart in a perfect vacuum. One ampere is the
current which, if it's flowing in these conductors, creates
between them a force of 0.2 micronewtons
per meter of length. (You're welcome to object that no one can
make an infinitely long conductor, nor a perfect vacuum. But
scientists can use the idealized definition to construct
appropriate realworld equipment in their laboratories.) The other
electrical units are all defined in terms of the ampere. For
example, one ampere represents a current flow of one coulomb
of charge per second. One ampere of current results from a
potential distribution of one volt
per ohm of resistance, or from a
power production rate of one watt
per volt of potential. The unit is known informally as the
amp, but A is its official symbol.
 Temperature
 kelvin (K)  Previously called the degree Kelvin (°K). One kelvin represents the same temperature difference as one degree Celsius. In 1967 the General Conference on Weights and Measures defined the temperature of the triple point of water (the temperature at which water exists simultaneously in the gaseous, liquid, and solid states) to be exactly 273.16 kelvins. Since this temperature is also equal to 0.01 ”ĘC, the temperature in kelvins is always equal to 273.15 plus the temperature in degrees Celsius. The kelvin equals exactly 1.8 degrees Fahrenheit. The unit is named for the British mathematician and physicist William Thomson (18241907), later known as Lord Kelvin after he was named Baron Kelvin of Largs.
 Amount of substance
 mole (mol)  The amount of a substance (as distinct from its mass or weight). Moles measure the actual number of atoms or molecules in an object. An alternate name is gram molecular weight, because one mole of a chemical compound is the same number of grams as the molecular weight of a molecule of that compound measured in atomic mass units. The official definition, adopted as part of the SI system in 1971, is that one mole of a substance contains just as many elementary entities (atoms, molecules, ions, or other kinds of particles) as there are atoms in 12 grams of carbon12 (carbon12 is the most common atomic form of carbon, consisting of atoms having 6 protons and 6 neutrons). The actual number of "elementary entities" in a mole is called Avogadro's number after the Italian chemist and physicist Amedeo Avogadro (17761856). Careful measurement determines Avogadro's number to be approximately 602.214 199 x 10^{21}. In the American system of naming big numbers, that's 602 sextillion 214 quintillion 199 quadrillion, give or take about 50 quadrillion.
 Pressure
 Pascal (Pa) 
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.
 Intensity of light.
 candela (cd)  Candela is
the Latin word for "candle." The unit has a long and complicated
history. Originally, it represented the intensity of an actual
candle, assumed to be burning whale tallow at a specified rate in
grains per hour. Later this definition was replaced with a
definition in terms of the light produced by the filament of an
incandescent light bulb. Still later a standard was adopted that
defined the candela as the intensity of 1/600 000 square meter of
a "black body" (a perfect radiator of energy) at the temperature
of freezing platinum (2042 K) and a pressure of 1 atmosphere.
This definition has also been discarded, and the candela is now
defined to be the luminous intensity of a light source producing
singlefrequency light at a frequency of 540 terahertz
(THz) with a power of 1/683 watt per
steradian, or 18.3988
milliwatts over a complete sphere centered at the light source.
The frequency of 540 THz corresponds to a wave length of
approximately 555.17 nanometers (nm); normal human eyes are more
sensitive to the yellowgreen light of this wavelength than to any
other. In order to produce 1 candela of singlefrequency light of
wavelength l, a lamp would have to radiate
1/(683V(l)) watts per steradian, where
V(l) is the relative sensitivity of the eye at
wavelength l. Values of V(l), defined by the
International Commission on
Illumination (CIE), are available online from the Color
and Vision Research Laboratories of the University of
California at San Diego and the University of Tübingen,
Germany.
Only the U.S. and a few of it's Caribbean neighbors are still on the English (or U.S.) Customary Weights and Measures [inch, foot, yard, mile, gallon, pound, ...].
It is also referred to as the Imperial system (sometimes referred to as footpoundsecond).
It was first defined in the English Weights and Measures Act of 1824.
The U.S. system is generally the same except for the U.S. gallon and bushel which are different than the Imperial gallon and bushel.
(I think Canada and the U.K. have converted to liters for measuring gas/petrol now.)
Use of Period:
Style guides are divided about whether you need to put a period after the abbreviations for English units of measure like feet, inches, and poundsą. So it's up to you to pick a style and use it consistently. In general, it's more common to use periods in the U.S. than in Britain (1). With the metric system, or more formally the International System of Units, you never use a period after the abbreviations (2).
Source: Grammar Girl
See also: Abbreviations at grammar.ccc.commnet.edu
See Also:
Size in reference here
Conversion tables here
ANSI Abbreviations for Scientific and Engineering Terms
Units of Measurement at Russ Rowlett's page at the UNC
Correct SImetric usage at Colorado State
Equivalents and Abbreviations at Georgia And Ft. Valley State
Measurement Abbreviations and Conversions  For Dummies
Abbreviations at grammar.ccc.commnet.edu
Cooking Measurement at Science of Cooking
last updated 5 Jan 2007
