Metric Prefixes
To help the SI units apply to a wide range of phenomena, the 19th General Conference on Weights and
Measures in 1991 extended the list of metric prefixes so that it reaches from yotta- at 1024 (one septillion) to
yocto- at 10-24 (one septillionth). Here are the metric prefixes, with their numerical equivalents stated in the
American system for naming large numbers:
giga- (G-)
109
1 billion
mega- (M-)
106
1 million
kilo- (k-)
103
1 thousand
hecto- (h-)
102
1 hundred
deka- (da-)** 10
1 ten
deci- (d-)
10-1
1 tenth
centi- (c-)
10-2
1 hundredth
milli- (m-)
10-3
1 thousandth
micro- (µ-)
10-6
1 millionth
nano- (n-)
10-9
1 billionth
pico- (p-)
10-12 1 trillionth
Notes:
I am often asked about prefixes for other multiples, such as 104, 105, 10-4, and 10-5. The prefix myria- (my-) was
formerly used for 104, but it is now considered obsolete and it is not accepted in the SI. To the best of my
knowledge, no prefixes were ever accepted generally for 105, 10-4, or 10-5.
There is a widespread misconception that prefixes for positive powers of ten are all capitalized, leading to the
use of K- for kilo- and D- for deca-. Although this does seem like a useful idea, it is not correct.
**The SI Brochure spelling of this prefix is deca-, but the U.S. National Institute of Standards and Technology
(NIST) recommends deka-. National variations in spelling of the prefixes are allowed by the SI. In Italian, for
example, hecto- is spelled etto- and kilo- is spelled chilo-. The symbols, however, are the same in all languages,
so dam (not dkm) is the symbol for the dekameter and km is the symbol for the Italian chilometro.
The prefixes hecto-, deka-, deci-, and centi- are widely used in everyday life but are generally avoided in
scientific work. Contrary to the belief of some scientists, however, the SI does allow use of these prefixes.
The last letter of a prefix is often omitted if the first letter of the unit name is a vowel, causing the combination
to be hard to pronounce otherwise. Thus 100 ares is a hectare and 1 million ohms is a megohm. However, the
last letter of the prefix is not omitted if pronunciation is not a problem, as in the case of the milliampere. The
letter "l" is sometimes added to prefixes before the erg, so 1 million ergs is a megalerg (sounds odd, but better
than "megerg").