SI base unit: metre (m)
The metre, symbol 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 m s–1, where the second is defined in terms of the caesium frequency ΔνCs.
![](/documents/20126/42149531/SI-colour-m.jpg/b9dd3d16-dd45-9ece-e19b-40b6a16dc40f?t=1594242137734)
This definition implies the exact relation c =
![](/documents/20126/42149531/SI-equation-m.jpg/431c26af-1e8e-9b1b-5d13-6e70732c8f4a?t=1594242458731)
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
See:
SI Brochure - Appendix 2
SI Brochure - excerpts from Appendix 4