SI Prefixes

Metric prefixes in everyday use
Text	Symbol	Factor	Power
exa	E	7018100000000000000♠1000000000000000000	1018
peta	P	7015100000000000000♠1000000000000000	1015
tera	T	7012100000000000000♠1000000000000	1012
giga	G	7009100000000000000♠1000000000	109
mega	M	7006100000000000000♠1000000	106
kilo	k	7003100000000000000♠1000	103
hecto	h	100	102
deca	da	10	101
(none)	(none)	1	100
deci	d	0.1	10−1
centi	c	0.01	10−2
milli	m	0.001	10−3
micro	μ	6994100000000000000♠0.000001	10−6
nano	n	6991100000000000000♠0.000000001	10−9
pico	p	6988100000000000000♠0.000000000001	10−12
femto	f	6985100000000000000♠0.000000000000001	10−15
atto	a	6982100000000000000♠0.000000000000000001	10−18

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 non-metric 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.

The BIPM specifies twenty prefixes for the International System of Units (SI).

Each prefix name has a symbol that 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 the SI symbols for kilometre, kilogram, and kilowatt, respectively. Where Greek letters are unavailable, the symbol for micro 'µ' is commonly replaced by 'u'.

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 are commonly used for everyday purposes, and the centimetre (cm) is especially 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".

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