How Many? A Dictionary of Units of Measurement
© Russ Rowlett and the University of North Carolina at Chapel Hill

Table of Contents
About the Dictionary
Using the Dictionary

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:

yotta- (Y-)


1 septillion

zetta- (Z-)


1 sextillion

exa- (E-)


1 quintillion

peta- (P-)


1 quadrillion

tera- (T-)


1 trillion

giga- (G-)


1 billion

mega- (M-)


1 million

kilo- (k-)


1 thousand

hecto- (h-)


1 hundred

deka- (da-)**


1 ten

deci- (d-)


1 tenth

centi- (c-)


1 hundredth

milli- (m-)


1 thousandth

micro- (µ-)


1 millionth

nano- (n-)


1 billionth

pico- (p-)


1 trillionth

femto- (f-)


1 quadrillionth

atto- (a-)


1 quintillionth

zepto- (z-)


1 sextillionth

yocto- (y-)


1 septillionth



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").

Binary prefixes

In computing, a custom arose of using the metric prefixes to specify powers of 2. For example, a kilobit is usually 210 = 1024 bits instead of 1000 bits. This practice leads to considerable confusion. In an effort to eliminate this confusion, in 1998 the International Electrotechnical Commission approved new prefixes for the powers of 2. These prefixes are as follows:



210 = 1 024



220 = 1 048 576



230 = 1 073 741 824



240 = 1 099 511 627 776



250 = 1 125 899 906 842 624



260 = 1 152 921 504 606 846 976

The Commission's ruling is that the metric prefixes should be used in computing just as they are used in other fields. Thus, 5 gigabytes (GB) should mean exactly 5 000 000 000 bytes, and 5 gibibytes (GiB) should mean exactly 5 368 709 120 bytes.


Return to the Dictionary Contents page.



April 16, 2005