Standard systems of number prefixes.
The standard system of prefixes for multiples is that laid down in SI units (Système International D’Unités), an international agreement dating from 1960, which defines standard units for quantities and the names for the decimal prefixes to use with them.
The standard SI multiples:
|deca-||10||Greek deka, ten|
|hecto-||100||Greek hekaton, hundred|
|kilo-||103||Greek khilioi, thousand|
|mega-||106||Greek megas, great|
|giga-||109||Greek gigas, giant|
|tera-||1012||Greek teras, monster|
|peta-||1015||Greek penta-, five, this being the fifth prefix in the series, by analogy with tera-|
|exa-||1018||Based on the Greek prefix hexa-, six, by deleting the first letter|
|zetta-||1021||Adapted from the Italian setta, seven|
|yotta-||1024||Adapted from the Italian otto, eight, with the last letter changed to match that of the other prefixes|
The standard SI submultiples:
|deci-||one-tenth or 10-1||Latin decimus, a tenth|
|centi-||one-hundredth or 10-2||Latin centum, a hundred|
|milli-||10-3||Latin mille, thousand|
|micro-||10-6||Greek mikros, small|
|nano-||10-9||Greek nanos, dwarf|
|pico-||10-12||Spanish pico, literally a little bit|
|femto-||10-15||Danish or Norwegian femten, fifteen|
|atto-||10-18||Danish or Norwegian atten, eighteen|
|zepto-||10-21||Adapted from septi-, seven, on the pattern of other multiples|
|yocto-||10-24||Similarly adapted from octo-, eight|
The prefixes hecto-, deca-, deci-, and centi- are generally avoided in scientific work.
Binary multiples for computer purposes
The use of decimal prefixes to describe the similar — but not identical — binary multiples used in computing (such as megabyte or terabit) has caused confusion — as a result of various conventions, a megabyte can be 1,048,576, 1,024,000, or 1,000,000. In 1998 the International Electrotechnical Commission (IEC) agreed an international standard for a new group of prefixes that removes the ambiguity; names use the first two letters of the SI decimal prefix, followed by the letters bi, for binary. These are slowly coming into use.