SI prefixes and their etymologies

Prefixes

Prefix	Symbol	Factor	Ordinary Notation*		Name
quetta	Q	1030	1 000 000 000 000 000 000 000 000 000 000		nonillion
ronna	R	1027	1 000 000 000 000 000 000 000 000 000		octillion
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
deca†	da	101	10		ten
		100	1		one, unity
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
ronto	r	10−27	0	.000 000 000 000 000 000 000 000 001	octillionth
quecto	q	10−30	0	.000 000 000 000 000 000 000 000 000 001	nonillionth

Notes

^* The “ordinary notation” column uses thin spaces to separate numbers into groups of three digits. In the US, commas are more often used for this purpose, but thin spaces have the advantage of avoiding confusion when numbers might be seen by readers who use the comma as the decimal marker. The practice of using commas as three-digit separators is considered incorrect for this very reason, and the use of thin spaces for that purpose is encouraged by the BIPM.
^† These prefixes are rarely used and not preferred, as they do not conform to the pattern of being multiples of 1000 [(10³)ⁿ]. See Is it OK to use centimetres? in the FAQ for more details.

Etymologies

Prefix	Pronunciation	Origin
quetta	KET-tuh	Latin: “ten” — (103)10
ronna	RON-uh	Greek: “nine” — (103)9
yotta	YOT-tuh or YOTE-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	GIG-uh or JIG-uh	Greek: “giant”
mega	MEG-uh	Greek: “big”
kilo	KILL-oh (not ki-LAH)	Greek: “thousand”
hecto	HECK-toe	Greek: “hundred”
deca	DECK-uh	Greek: “ten”
deci	DESS-ih	Latin: “tenth”
centi	SENT-ih	Latin: “hundredth”
milli	MILL-ih	Latin: “thousandth”
micro	MY-crow	Greek: “small”
nano	NA-noh	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
ronto	RON-toe	Greek: “nine” — (103)−9
quecto	KEK-toe	Latin: “ten” — (103)−10

Last updated: 2022-11-19