SI prefixes for SI units are:

^{†} In Britain and Ireland the names that end in "-iard" and "-iardth" are not used. Instead for example 10^{9} is referred to as a "thousand million", though the U.S. naming is also sometimes used.

(Sub)multiple | Prefix | Symbol | Name (US and Canada) | Name (Europe)^{†} |
---|---|---|---|---|

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

10^{21} | zetta | Z | Sextillion | Trilliard |

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

10^{15} | peta | P | Quadrillion | Billiard |

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

10^{9} | giga | G | Billion | Milliard |

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

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

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

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

10^{0} | iso | i | Unit | Unit |

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

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

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

10^{-6} | micro | μ | Millionth | 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 dm = 5 x 10
^{-2}m = 5 x 0.01 m = 0.05 m - 3 MW = 3 x 10
^{6}W = 3 x 1 000 000 W = 3 000 000 W

Note that when used with non-SI units, prefices *occasionally* have other values. For instance, when referring to units of storage like bytes, *kilo* means 2^{10}=1024 rather than 1000.

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? (being equivalent to 1m^{3}), and 1 dm^{3} (equivalent to one litre, sometimes referred to as a 'dumphrey').