Contents: Units | Prefixes (kilo, nano, ...) | Usage Rules | Abbreviations | Definitions

Measurement (Quantities, size, units)

Système International d'Unités (International System of Units) (SI) is maintained by a small agency in Paris, the International Bureau of Weights and Measures (BIPM, for Bureau International des Poids et Mesures), and it is updated every few years by an international conference, the General Conference on Weights and Measures (CGPM, for Conférence Générale des Poids et Mesures), attended by representatives of all the industrial countries and international scientific and engineering organizations. The 22nd CGPM met in October 2003; the next meeting will be in 2007.

• the meter for distance,
• the kilogram for mass,
• the second for time,
• the ampere for electric current,
• the kelvin for temperature,
• the mole for amount of substance
• the candela for intensity of light.

The US still uses Inch-Pound (I-P) Units
See: A Dictionary of Units of Measurement by Russ Rowlett at the University of North Carolina.
SI conversion Factors | NIST

Distance:
centimeter cm
meter *    m
kilometer  km
foot       ft or '
inch       in or "
yard       yd

Area:
square meter m2
square foot  ft2 †
square yard  yd2 †
square mile  mi2 †
acre         ac or A

Volume:
liter *    L
gallon     gal
quart      qt
cup        c or C
ounce      oz or fl oz
tablespoon tbsp, tblsp, or Tsp
teaspoon   tsp
cubic foot ft3 or cu ft
acre foot  ac ft or af

Weight-Force:
gram    g
Newton  N
pound   lb or #
pound-force lbf
pound-mass  lbm

Angle
degree  ° [°]
minute (plane angle)  '
second (plane angle)  "
radian  rad

Time:
minute  min or ' or m
hour    h
second  s

Speed:
Miles per hour      mi/h or mph
Foot per second     ft/s or ft/sec or fps
Kilometers per hour km/h
Meters per second   m/s
Knot                kn or kt
revolutions per minute r/min or rpm 

Electrical:
alternating current    ac
ampere                 A
ampere • hour          Ah
baud                   Bd
Direct current         dc
hertz                  Hz
ohm                    Ω (Omega) 
inductance-capacitance LC
resistance-capacitance RC
volt                   V
Watt                   W

Temperature:
degree             deg
degree Celsius     °C
degree Fahrenheit  °F
kelvin              K

light
candella   cd
lux        lx
lumen      lm

• 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)

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:

• 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 case-sensitive 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. km², NOT sq km.
• A raised dot (also called a middle dot or half-high 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/s² 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.

There is also a special unit for length called angström, which is 1E-10 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.

Distance

meter (m) - Originally, the meter was designed to be one ten-millionth 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 well-equipped 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. Light-year, AU (Astronomical Unit), Parsec,

Mass

kilogram (kg) - The kilogram is defined as the mass of the standard kilogram, a platinum-iridium 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 cesium-133 equal to 9 192 631 770 hertz (cycles per second). In other words, if we really want to measure a second, we count 9 192 631 770 cycles of this radiation. This definition allows scientists to reconstruct the second anywhere in the world with equal precision.

Electric current

ampere (A)- named for the French physicist André-Marie Ampère (1775-1836), 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 real-world 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 (1824-1907), 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 carbon-12 (carbon-12 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 (1776-1856). Careful measurement determines Avogadro's number to be approximately 602.214 199 x 10²¹. 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 single-frequency 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 yellow-green light of this wavelength than to any other. In order to produce 1 candela of single-frequency 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 foot-pound-second). 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 SI-metric 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