An
SI prefix (also known as a
metric prefix) is a name or associated symbol that precedes a basic unit of measure (or its symbol) to form a
decimal multiple or submultiple. The abbreviation
SI is from the French language name
Système International d’Unités (also known as
International System of Units). SI&_160;
prefixes are used to reduce the number of zeros shown in numerical quantities. For example, one-billionth of an ampere (a small electrical current) can be written as
0.000000001ampere. In symbol form, this is written as
0.000000001A. Using an SI&_160;prefix, this is equivalent to 1&_160;
nanoampere or 1&_160;nA. The SI&_160;prefixes are standardized by the
Bureau International des Poids et Mesures (BIPM, also known as the
International Bureau of Weights and Measures) and are the product of
four&_160;resolutions dating from 1960 to 1991.
The twenty SI prefixes are shown in the chart below.
Twenty SI prefixes are available to combine with units of measure. For example, the prefix kilo- denotes a multiple of one thousand, so 1 kilometre equals 1000 metres, 1 kilogram equals 1000 grams, 1 kilowatt equals 1000 watts, and so on. Each SI prefix name has an associated symbol which can be used in combination with the symbols for units of measure. Thus, the "kilo-" symbol, k, can be used to produce km, kg, and kW, (kilometre, kilogram, and kilowatt). SI prefixes are internationally recognized and also exist outside the SI (many of them long pre-date SI, going back to the original introduction of the metric system); prefixes may also be used in combination with non-SI units; for example milligauss (mG), kilofoot (kft) and microinch (µin).
Prefixes may not be used in combination. This even applies for mass, for which the SI base unit (which is the kilogram, not the gram) already contains a prefix. So milligram (mg) is used instead of microkilogram (µkg), for example.
