Resolution 2 of the 15th CGPM (1975)
Recommended value for the speed of light

The 15th Conférence Générale des Poids et Mesures,

considering the excellent agreement among the results of wavelength measurements on the radiations of lasers locked on a molecular absorption line in the visible or infrared region, with an uncertainty estimated at plus-minus 4 x 10–9 which corresponds to the uncertainty of the realization of the metre,

considering also the concordant measurements of the frequencies of several of these radiations,

recommends the use of the resulting value for the speed of propagation of electromagnetic waves in vacuum c = 299 792 458 metres per second.

Comptes rendus de la 15e CGPM (1975), 1976, 103
Metrologia, null, 11, 4, 179-183


The relative uncertainty given here corresponds to three standard deviations in the data considered.