SI prefixes and their etymologies

SI prefixes

Prefix	Symbol	Factor	Ordinary Notation*	Name
yotta	Y	10²⁴	1 000 000 000 000 000 000 000 000	septillion**
zetta	Z	10²¹	1 000 000 000 000 000 000 000	sextillion**
exa	E	10¹⁸	1 000 000 000 000 000 000	quintillion**
peta	P	10¹⁵	1 000 000 000 000 000	quadrillion**
tera	T	10¹²	1 000 000 000 000	trillion**
giga	G	10⁹	1 000 000 000	billion**
mega	M	10⁶	1 000 000	million
kilo	k	10³	1 000	thousand

hecto	h	10²	100	hundred
deka	da	10¹	10	ten

		10⁰	1	one

deci	d	10^-1	0.1	tenth
centi	c	10^-2	0.01	hundredth

milli	m	10^-3	0.001	thousandth
micro	μ	10^-6	0.000 001	millionth
nano	n	10^-9	0.000 000 001	billionth**
pico	p	10^-12	0.000 000 000 001	trillionth**
femto	f	10^-15	0.000 000 000 000 001	quadrillionth**
atto	a	10^-18	0.000 000 000 000 000 001	quintillionth**
zepto	z	10^-21	0.000 000 000 000 000 000 001	sextillionth**
yocto	y	10^-24	0.000 000 000 000 000 000 000 001	septillionth**

^*	The “ordinary notation” column uses spaces to separate numbers into groups of three digits. In the US, commas are more often used for this purpose, but spaces have the advantage of avoiding confusion when numbers might be seen by readers who use the comma as the decimal point.
**	The terms billion, trillion, etc., can be ambiguous. The terms are used here in the American English sense, but British English traditionally defined billion as a million million, rather than a thousand million. However, recent British usage tends to match the American meanings.

SI prefix etymologies

Prefix	Pronunciation	Origin
yotta	YOTE-uh or YOTT-uh	Latin: “eight” [(10³)⁸]
zetta	ZETT-uh	Latin: “seven” [(10³)⁷]
exa	EX-uh	Greek: “six” [(10³)⁶]
peta	PET-uh	Greek: “five” [(10³)⁵]
tera	TAIR-uh	Greek: “monster”
giga	JIG-uh or GIG-uh	Greek: “giant”
mega	MEG-uh	Greek: “big”
kilo	KILL-oh	Greek: “thousand”
hecto	HECK-toe	Greek: “hundred”
deka	DECK-uh	Greek: “ten”

deci	DESS-ih	Latin: “tenth”
centi	SENT-ih	Latin: “hundredth
milli	MILL-ih	Latin: “thousandth”
micro	MIKE-roe	Greek: “small”
nano	NAN-oh	Greek: “dwarf”
pico	PEEK-oh	Spanish: “tiny bit”
femto	FEM-toe	Dano-Norwegian: “fifteen”
atto	AT-toe	Dano-Norwegian: “eighteen”
zepto	ZEP-toe	Latin: “seven” [(10³)^-7]
yocto	YOCK-toe	Latin: “eight” [(10³)^-8]

These tables were provided by David Bernklau, Certified Metrication Specialist (CMS), The first table is an expanded version of Table 2 published in Metric in Minutes by Dennis Brownridge.

Back to USMA home.