Definition: 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 Δν_{Cs}.

This definition implies the exact relation \(c = {\rm{299\ 792\ 458\ m\ s}^{-1}}\). Inverting this relation gives an exact expression for the metre in terms of the defining constants \(c\) and \(\Delta\nu_{\rm{Cs}}\):
$$1\ {\rm{m}} = ( \frac{c}{299\:792\:458} )\ {\rm{s}} = \frac{9\:192\:631\:770}{299\:792\:458} \frac{c}{\Delta\nu_{\rm{Cs}}} \approx 30.663\:319 \frac{c}{\Delta\nu_{\rm{Cs}}}$$

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.