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metre (m)

Document v1 · published by Koskamo · updated 2026-10-05

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The metre (symbol m) is the SI base unit of length, per the BIPM SI Reference Point.

  • Name: metre
  • Symbol: m
  • Unit type: SI base unit
  • Quantity kind: length
  • Definition (status current, in force since 2019-05-20): The metre, symbol $\rm{m}$, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum, $c$, to be $299\:792\:458$ when expressed in the unit ${\rm{m}}\ {{\rm{s}}^{-1}}$, where the second is defined in terms of the caesium frequency $\Delta\nu_{\rm{Cs}}$.
  • Defining constant: speed of light (fixed numerical value 299 792 458)
  • Defining equation: 1\;{\rm{m}} = \left(\frac{c}{299\:792\:458}\right) {\rm{s}} = \frac{{9\:192\:631\:770}}{299\:792\:458} \frac{c}{\Delta\nu_{\rm{Cs}}} \approx {30.663\:319} \frac{c}{\Delta\nu_{\rm{Cs}}}
  • Defining resolution: https://si-digital-framework.org/bodies/CGPM#CGPM26-Res1
  • Superseded definitions: metre1889 (1889-05-20 to 1927-09-29); metre1927 (1927-09-30 to 1960-10-13); metre1960 (1960-10-14 to 1983-10-16); metre1983 (1983-10-17 to 2019-05-19)

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si-digital-framework.org
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BIPM SI Reference Point (si-digital-framework.org), the SI defining constants, base and derived units, and prefixes; CC BY 3.0 IGO

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  1. Published by Koskamo

    Version 1