SI prefixes for decimal multiples

There are 20 internationally accepted prefixes to denote 10ⁿ multiples of units. 10 of these prefixes denote (n>0) multiples, and other 10 denote (n<0) submultiples:

SI Prefixes for decimal multiples
Number	Factor	Name	Symbol
1 000 000 000 000 000 000 000 000	10²⁴	yotta	Y
1 000 000 000 000 000 000 000	10²¹	zetta	Z
1 000 000 000 000 000 000	10¹⁸	exa	E
1 000 000 000 000 000	10¹⁵	peta	P
1 000 000 000 000	10¹²	tera	T
1 000 000 000	10⁹	giga	G
1 000 000	10⁶	mega	M
1 000	10³	kilo	k
100	10²	hecto	h
10	10¹	deca	da
0.1	10^-1	deci	d
0.01	10^-2	centi	c
0.001	10^-3	milli	m
0.000 001	10^-6	micro	µ
0.000 000 001	10^-9	nano	n
0.000 000 000 001	10^-12	pico	p
0.000 000 000 000 001	10^-15	femto	f
0.000 000 000 000 000 001	10^-18	atto	a
0.000 000 000 000 000 000 001	10^-21	zepto	z
0.000 000 000 000 000 000 000 001	10^-24	yocto	y

Each prefix is represented by its own symbol. The symbols are case sensitive. Thus, m means milli and M means mega.

Prefixes ranging from milli to kilo were first introduced in 1793 by the French Academy of Sciences under direction of French National Assembly as part of the Metric System. Greek prefixes were used for multiples of 10 and Latin prefixes for submultiples of 10. In 1874 BAAS adapted these prefixes as part of their CGS system, and added prefixes micro and mega. Later, 12 prefixes ranging from pico to tera were defined as part of the International System of Units — SI, which was adopted in 1960. SI is maintained by BIPM under exclusive supervision of CIPM and resolutions made by CGPM. Further 8 prefixes were added to SI in years 1964 (femto, atto), 1975 (peta, exa) and 1991 (zetta, zepto, yotta, yocto).

The prefixes have the following etymology:

exa — alteration of hexa from Greek hex meaning “six” (the sixth power of 10³)
peta — alteration of penta from Greek pente meaning “five” (the fifth power of 10³)
tera — from Greek teras meaning “monster”
giga — from Greek gigas meaning “giant”
mega — from Greek megas meaning “great”
kilo — from Greek khilioi meaning “thousand”
hecto — French, alteration of Greek hekaton meaning “hundred”
deca — from Greek deka meaning “ten”
deci — from Latin decimus meaning tenth, from decem meaning “ten”
centi — from Latin centi-, from centum meaning “hundred”
milli — from Latin mille meaning “thousand”
micro — from Greek mikros meaning “small”
nano — from Greek nannos meaning “dwarf”
pico — from Spanish pico meaning “small quantity”
femto — from Danish, or Norwegian word femten meaning “fifteen”
atto — from Danish, or Norwegian word atten meaning “eighteen”

The names zepto and zetta are derived from septo, from Latin septem which means seven (the seventh power of 10³) and the letter ‘z’ is substituted for the letter ‘s’ to avoid the duplicate use of the letter ‘s’ as a symbol in SI. The names yocto and yotta are derived from Latin octo which means eight (the eighth power of 10³); the letter ‘y’ is added to avoid the use of the letter ‘o’ as a symbol because it may be confused with the number zero. The CGPM has decided to name the prefixes, starting with the seventh power of 10³, with the letters of the Latin alphabet, but starting from the end. Therefore the choice of letters ‘z’ and ‘y’. The initial letter ‘h’ of the word hexa in standard French is silent, so it was removed in order to simplify things.