An **SI prefix** is a prefix which can be applied to any unit of the **International System of Units** (SI) to give subdivisions and multiples of that unit.

For example, the prefix "kilo" multiplies by one thousand, so a *kilo*metre is 1,000 metres, and a *kilo*watt is 1,000 watts. The prefix "milli" subdivides by a thousand, so a *milli*metre is one thousandth of a metre (1,000 millimetres in a metre), and a *milli*litre is one thousandth of a litre. The ability to apply the same prefixes to any SI unit is one of the key strengths of the SI, since it considerably simplifies the system's learning and use.

The most commonly used prefixes include:

- giga = 10
^{9}, US billion or European milliard, a thousand million - mega = million
- kilo = thousand
- centi = one hundredth
- milli = one thousandth

(Sub)multiple | Prefix | Symbol | Name (Americas) | Name (European) |
---|---|---|---|---|

10^{24} | yotta | Y | Septillion | Quadrillion |

10^{21} | zetta | Z | Sextillion | Thousand trillion (Trilliard) |

10^{18} | exa | E | Quintillion | Trillion |

10^{15} | peta | P | Quadrillion | Thousand billion (Billiard) |

10^{12} | tera | T | Trillion | Billion |

10^{9} | giga | G | Billion | Thousand million (Milliard) |

10^{6} | mega | M | Million | |

10^{3} | kilo | k | Thousand | |

10^{2} | hecto | h | Hundred | |

10^{1} | deca or deka | da | Ten | |

10^{-1} | deci | d | Tenth | |

10^{-2} | centi | c | Hundredth | |

10^{-3} | milli | m | Thousandth | |

10^{-6} | micro | μ | Millionth | |

10^{-9} | nano | n | Billionth | Milliardth |

10^{-12} | pico | p | Trillionth | Billionth |

10^{-15} | femto | f | Quadrillionth | Billiardth |

10^{-18} | atto | a | Quintillionth | Trillionth |

10^{-21} | zepto | z | Sextillionth | Trilliardth |

10^{-24} | yocto | y | Septillionth | Quadrillionth |

Examples:

- 5 cm = 5 × 10
^{-2}m = 5 × 0.01 m = 0.05 m - 3 MW = 3 × 10
^{6}W = 3 × 1 000 000 W = 3 000 000 W

Prefixes where the exponent is divisible by three are recommended. Hence '100 metres' rather than 'one hectometre'. Notable exceptions include centimetre, hectare (hecto-are), centilitre, and 1 dm^{3} (equivalent to one litre).

The accepted pronunciation of the initial G of "giga-" was once soft, /ˈdʒaɪgə/ (like "gigantic"), but now the hard pronunciation, /ˈgɪgə/, is probably more common.

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- K = 2
^{10}= 1,024 - M = 2
^{20}= 1,048,576 - G = 2
^{30}= 1,073,741,824 - T = 2
^{40}= 1,099,511,627,776 - P = 2
^{50}= 1,125,899,906,842,624.

This inconsistency did not seem relevant when computers had little storage and communication links were relatively slow, but the increasing capacity of computing systems and speed of network links began making this inconsistency a more serious problem.

Accordingly, the International Electrotechnical Commission adopted new binary prefixes in 1998, formed from the first syllable of the decimal prefix plus 'bi' (pronounced 'bee'). The symbol is the decimal symbol plus 'i'. So now, one kilobyte (1 kB) equals 1000 bytes, whereas one kibibyte (1 KiB) equals 2^{10} = 1024 bytes. Likewise mebi (2^{20}), gibi (2^{30}), tebi (2^{40}), pebi (2^{50}), and exbi (2^{60}).
For example, at 1 MB/s = 10^{6} bytes per second, it would take slightly longer than one second to transfer an object 1 MiB = 2^{20} bytes in size.
The adoption of these prefixes has been very limited.

For more information on these power-of-two prefixes, see Binary prefixes.

See also Orders of magnitude.

*This article (or an earlier version of it) contains material from FOLDOC, used with permission.*