The
International System of Units (SI) specifies a set of unit prefixes, known as
SI prefixes, also known as
metric prefixes. An SI
prefix is a name that precedes a basic unit of measure to indicate a
decimal multiple or
fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol. The SI&_160;prefixes are standardized by the
International Bureau of Weights and Measures in resolution dating from 1960 to 1991.
[1]SI prefixes are used to reduce the number of zeros shown in numerical quantities before or after the decimal point. For example, an electrical current of 0.000000001ampere, or one-billionth (short scale) of an ampere, is written by using the SI-prefix nano as 1&_160;nanoampere or 1&_160;nA.
The International System of Units specifies twenty SI prefixes
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).
