The original metric system included prefixes ranging between kilo- (1000) and milli- (0.001). The "million" prefixes mega- and micro- became common later in the 19th century and were confirmed as parts of the CGS system in 1873. Since the establishment of the International System of Units in 1960 there have been four extensions of the lsit of prefixes, the most recent being in 1991.

Here are the metric prefixes, with their numerical equivalents stated in the American system for naming large numbers and the dates of their approval.

yotta- (Y-) 10 ^{24}1 septillion 19th General Conference, 1991 zetta- (Z-) 10 ^{21}1 sextillion 19th General Conference, 1991 exa- (E-) 10 ^{18}1 quintillion 15th General Conference, 1975 peta- (P-) 10 ^{15}1 quadrillion 15th General Conference, 1975 tera- (T-) 10 ^{12}1 trillion 11th General Conference, 1960 giga- (G-) 10 ^{9}1 billion 11th General Conference, 1960 mega- (M-) 10 ^{6}1 million CGS System, 1873 kilo- (k-) 10 ^{3}1 thousand Original metric system, 1795 hecto- (h-) 10 ^{2}1 hundred Original metric system, 1795 deka- (da-)** 10 1 ten Original metric system, 1795 deci- (d-) 10 ^{-1}1 tenth Original metric system, 1795 centi- (c-) 10 ^{-2}1 hundredth Original metric system, 1795 milli- (m-) 10 ^{-3}1 thousandth Original metric system, 1795 micro- (µ-) 10 ^{-6}1 millionth CGS System, 1873 nano- (n-) 10 ^{-9}1 billionth 11th General Conference, 1960 pico- (p-) 10 ^{-12}1 trillionth 11th General Conference, 1960 femto- (f-) 10 ^{-15}1 quadrillionth 12th General Conference, 1964 atto- (a-) 10 ^{-18}1 quintillionth 12th General Conference, 1964 zepto- (z-) 10 ^{-21}1 sextillionth 19th General Conference, 1991 yocto- (y-) 10 ^{-24}1 septillionth 19th General Conference, 1991

**Notes:**

Before the expansion of the prefix list in 1960 there was a proliferation of **double prefixes** such as micromicro- for 10^{-12} and kilomega- for 10^{9}. The introduction of new prefixes made these constructions unnecessary, and the International System now prohibits them.

I am often asked about prefixes for other multiples, such as
10^{4}, 10^{5}, 10^{-4}, and 10^{-5}.
The prefix myria- (my-) for 10^{4} was included in the original metric system and dimi- (dm-), a contraction of decimilli-, was sometimes used for 10^{-4}, but these prefixes are now obsolete and are not accepted in the SI. To the
best of my knowledge, no prefixes were ever accepted generally for
10^{5}, 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").

In computing, a custom arose of using the metric prefixes to
specify powers of 2. For example, a kilobit was sometimes 2^{10}
= 1024 bits instead of 1000 bits. This practice led, and still 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:

kibi- Ki- 2 ^{10}= 1 024mebi- Mi- 2 ^{20}= 1 048 576gibi- Gi- 2 ^{30}= 1 073 741 824tebi- Ti- 2 ^{40}= 1 099 511 627 776pebi- Pi- 2 ^{50}= 1 125 899 906 842 624exbi- Ei- 2 ^{60}= 1 152 921 504 606 846 976

The Commission's ruling is that the standard metric prefixes should be used in computing just as they are used in other fields. Thus, 32 gigabytes (GB) means exactly 32 000 000 000 bytes, and 32 gibibytes (GiB) means exactly 34 359 738 368 bytes.

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All material in this folder is copyright 2018 by Russ Rowlett and the University of North Carolina at Chapel Hill. Permission is granted for personal use and for use by individual teachers in conducting their own classes. All other rights reserved. You are welcome to make links to this page, but please do not copy the contents of any page in this folder to another site. The material at this site will be updated from time to time.

Revised April 26, 2018