The Metric System - SI Prefixes

The Systèm International d'Unités (SI) is a system of units for the measurement of physical quantities that has been almost universally adopted (the U.S.A. and a few obscure countries still use older Imperial measurement systems). It uses prefixes to indicate multiples and submultiples in order to avoid a multiplicity of zeroes when expressing very large or very small amounts. Most of the names of the prefixes are derived from the Greek language, although several of the prefixes for submultiples are derived from Latin (e.g. deci-, centi-, milli-) or from other languages. The following is a list of prefixes along with the standard abbreviations and factors of the numbers:

Note that some of the very large and very small prefixes, as well as myria- and myrio-, are not official SI prefixes. These are given in italics.

Prefix	Abbreviation	Factor	Derivation
vendeka-	V	10 33 (decillion)	Greek
xenna-	X	10 27 (octillion)	Greek
yotta-	Y	10 24 (septillion)	Greek
zetta-	Z	10 21 (sextillion)	Latin
exa-	E	10 18 (quintillion)	Greek
peta-	P	10 15 (quadrillion)	Greek
tera-	T	10 12 (trillion)	Greek
giga-	G	10 9 (billion)	Greek
mega-	M	10 6 (million)	Greek
myria-	ma	10 4 (ten thousand)	Greek
kilo-	k	10 3 (thousand)	Greek
hecto-	h	10 2 (hundred)	Greek
deca-	da	10 1 (ten)	Greek
deci-	d	10 -1 (tenth)	Latin
centi-	c	10 -2 (hundredth)	Latin
milli-	m	10 -3 (thousandth)	Latin
myrio-	mo	10 -4 (ten-thousandth)	Greek
micro-	μ	10 -6 (millionth)	Greek
nano-	n	10 -9 (billionth)	Latin
pico-	p	10 -12 (trillionth)	Italian
femto-	f	10 -15 (quadrillionth)	Danish
atto-	a	10 -18 (quintillionth)	Danish
zepto-	z	10 -21 (sextillionth)	Latin
yocto-	y	10 -24 (septillionth)	Latin
xenno-	x	10 -27 (octillionth)	Greek
vendeko-	v	10 -33 (decillionth)	Greek

Note that the prefix for 10¹ (deca-) is often spelled deka- and can be found with an abbreviation of D in older sources.

Note that the hecto-, deca-, deci-, and centi- prefixes are typically used only if the milli- or kilo- prefixes would be impractical for measuring numbers used in the "real world". For example, we might say 28 centimetres instead of 280 millimetres. As well, the more extreme prefixes are rarely used (simply expressing the measurements using scientific notation is usually just as easy).

If you're interested in the Ancient Greeks (who had nothing to do with the metric system; the developers of the metric system just borrowed some of their words), see the Ancient Greek timeline.