# Millimetre

The **millimetre** (international spelling; SI unit symbol **mm**) or **millimeter** (American spelling) is a unit of length in the International System of Units (SI), equal to one thousandth of a metre, which is the SI base unit of length. Therefore, there are one thousand millimetres in a metre. There are ten millimetres in a centimetre.

millimetre | |
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General information | |

Unit system | SI |

Unit of | Length |

Symbol | mm |

Named after | The metric prefix mille (Latin for "one thousand") and the metre |

Conversions | |

1 mm in ... | ... is equal to ... |

micrometres | 1×10^{3} μm = 1000 μm |

centimetres | 1×10^{−1} cm = 0.1 cm |

metres | 1×10^{−3} m = 0.001 m |

kilometres | 1×10^{−6} km |

inches | 0.039370 in |

feet | 0.0032808 ft |

One millimetre is equal to 1000 micrometres or 1000000 nanometres. Since an inch is officially defined as exactly 25.4 millimetres, a millimetre is equal to exactly 5⁄127 (≈ 0.03937) of an inch.