
A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or fraction of the unit. While all metric prefixes in common use today are decadic, historically there have been a number of binary metric prefixes as well.〔 Each prefix has a unique symbol that is prepended to the unit symbol. The prefix ''kilo'', for example, may be added to ''gram'' to indicate ''multiplication'' by one thousand; one kilogram is equal to one thousand grams. The prefix ''milli'', likewise, may be added to ''metre'' to indicate ''division'' by one thousand; one millimetre is equal to one thousandth of a metre. Decimal multiplicative prefixes have been a feature of all forms of the metric system with six dating back to the system's introduction in the 1790s. Metric prefixes have even been prepended to nonmetric units. The SI prefixes are standardized for use in the International System of Units (SI) by the International Bureau of Weights and Measures (BIPM) in resolutions dating from 1960 to 1991. Since 2009, they have formed part of the International System of Quantities. ==List of SI prefixes== The BIPM specifies twenty prefixes for the International System of Units (SI). Each prefix name has a symbol which is used in combination with the symbols for units of measure. For example, the symbol for ''kilo'' is ''k'', and is used to produce ''km'', ''kg'', and ''kW'', which are kilometre, kilogram, and kilowatt, respectively. Prefixes may not be used in combination. This also applies to mass, for which the SI base unit (kilogram) already contains a prefix. For example, milligram (mg) is used instead of microkilogram (µkg). In arithmetic of measurements having prefixed units, the prefixes must be expanded to their numeric multiplier, except when adding or subtracting values with identical units. Hence, × = × = = . Prefixes corresponding to an integer power of one thousand are generally preferred. Hence ''100 m'' is preferred over ''1 hm'' (hectometre) or ''10 dam'' (decametres). The prefixes hecto, deca, deci, and centi were commonly used for everyday purposes, especially the centimetre (cm) is common. However, some modern building codes require that the millimetre be used in preference to the centimetre, because "use of centimetres leads to extensive usage of decimal points and confusion".〔https://web.archive.org/web/20111215115519/http://wbdg.org/ccb/GSAMAN/mdg.pdf〕 When units occur in exponentiation, for example, in square and cubic forms, the multiplication prefix must be considered part of the unit, and thus included in the exponentiation. * 1 km^{2} means one square kilometre, or the area of a square of by and not square metres. * 2 Mm^{3} means two cubic megametres, or the volume of two cubes of by by or , and not cubic metres (). ;Examples * = = = 0.05 m * = = = = = * 3 MW = = 3 × = 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Metric prefix」の詳細全文を読む スポンサード リンク
