SI base unit
Name Symbol Quantity
metre m length


The 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 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.

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 ΔνCs :

1 \mspace{4mu} \text{m} \mspace{10mu} = \dfrac{{c}} {299 \mspace{4mu} 792 \mspace{4mu} 458} \mspace{6mu} \text{s}\\ \\ \\ 1 \mspace{4mu} \text{m} \mspace{10mu} = \dfrac{9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770}{299 \mspace{4mu} 792 \mspace{4mu} 458} \mspace{6mu} \dfrac{c}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{m} \mspace{10mu} = 30.663 \mspace{4mu} 318 \mspace{4mu} 988 \mspace{4mu} 498 \mspace{4mu} 369 \mspace{4mu} 762 \text{...} \mspace{6mu} \dfrac{c}{\Delta \nu _{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 1299 792 458 of a second.