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 ± 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.

DOI : 10.59161/CGPM1975RES2E

The reader should note that the official version of this Resolution is the French text


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

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