**metre**

## SI base unit |
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Name | Symbol | Quantity | |

metre | m | length | |

## DefinitionThe metre, symbol m, is the SI base unit of length. The metre is defined by taking the fixed numerical value of the speed of light in vacuum |

The definition of the metre implies the exact relation *c* = 299 792 458 m s^{–1}. Inverting this relation gives an exact expression for the metre in terms of the defining constants *c* and Δ*ν*_{Cs} :

The effect of this definition is that one metre is the length of the path travelled by light in vacuum during a time interval with duration of ^{1}⁄_{299 792 458} of a second.