Unit prefix

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

Decimal Value SI 1000 k kilo 10002 M mega 10003 G giga 10004 T tera 10005 P peta 10006 E exa 10007 Z zetta 10008 Y yotta	Binary Value IEC JEDEC 1024 Ki kibi K kilo 10242 Mi mebi M mega 10243 Gi gibi G giga 10244 Ti tebi – 10245 Pi pebi – 10246 Ei exbi – 10247 Zi zebi – 10248 Yi yobi –

A unit prefix is a specifier or mnemonic that is prepended to units of measurement to indicate multiples or fractions of the units. Units of various sizes are commonly formed by the use of such prefixes. The prefixes of the metric system, such as kilo and milli, represent multiplication by powers of ten. In information technology it is common to use binary prefixes, which are based on powers of two. Historically, many prefixes have been used or proposed by various sources, but only a narrow set has been recognised by standards organisations.

The prefixes of the metric system precede a basic unit of measure to indicate a decadic multiple and fraction of a unit. Each prefix has a unique symbol that is prepended to the unit symbol. Some of the prefixes date back to the introduction of the metric system in the 1790s, but new prefixes have been added, and some have been revised. The International Bureau of Weights and Measures has standardised twenty metric prefixes in resolutions dating from 1960 to 1991 for use with the International System of Units (SI).

Although formerly in use, the SI disallows combining prefixes; the microkilogram or centimillimetre, for example, are not permitted. Prefixes corresponding to powers of one thousand are usually preferred, however, units such as the hectopascal, hectare, decibel, centimetre, and centilitre, are commonly used. In mathematical contexts, the unit prefixes are always considered part of the unit, so that, e.g., in exponentiation, 1 km² means one square kilometre not one thousand square metre and 1 cm³ means one cubic centimetre not one hundredth of a cubic metre.

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