Metric unit

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

Variants of the metric system

Quantity CGS MKS MTS distance, displacement, length, height, etc. (d, x, l, h, etc.) centimetre (cm) metre (m) metre mass (m) gram (g) kilogram (kg) tonne (t) time (t) second (s) second second speed, velocity (v, v) cm/s m/s m/s acceleration (a) gal (Gal) m/s2 m/s2 force (F) dyne (dyn) newton (N) sthene (sn) pressure (P or p) barye (Ba) pascal (Pa) pièze (pz) energy (E, Q, W) erg (erg) joule (J) kilojoule (kJ) power (P) erg/s watt (W) kilowatt (kW) viscosity (µ) poise (p) Pa·s pz·s

The metric system is an internationally agreed decimal system of measurement. It was originally based on the mètre des Archives and the kilogramme des Archives introduced by the French First Republic in 1799, but over the years the definitions of the metre and the kilogram have been refined, and the metric system has been extended to incorporate many more units. Although a number of variants of the metric system emerged in the late nineteenth and early twentieth centuries, the term is now often used as a synonym for "SI" or the "International System of Units"—the official system of measurement in almost every country in the world.

The metric system has been officially sanctioned for use in the United States since 1866, but the U.S.A. remains the only industrialised country that has not fully adopted the metric system as its official system of measurement (although, in 1988, the United States Congress passed the Omnibus Foreign Trade and Competitiveness Act, which designates "the metric system of measurement as the preferred system of weights and measures for U.S. trade and commerce"). Among many other things, the act requires federal agencies to use metric measurements in nearly all of their activities, although there are still exceptions allowing traditional linear units to be used in documents intended for consumers. Many sources also cite the countries of Liberia and Myanmar (Burma) as the only other countries not to have done so. Although the United Kingdom also uses the metric system for most administrative and trade purposes, imperial units are widely used by the public and are permitted or obligatory for some purposes, such as road signs.

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