The speed of light in vacuum

SI defining constant
Name Symbol Value Unit Expressed in terms of SI base units
the speed of light in vacuum c 299 792 458 m s−1 m s−1

Definition

The speed of light in vacuum, symbol c, is a fundamental physical constant. It is the speed at which all electromagnetic radiation travels in vacuum.

The numerical value of the speed of light in vacuum, symbol c, is defined to be exactly 299 792 458 when expressed in the unit metre per second, m s−1.

The speed of light is the same regardless of the motion of the source or the inertial reference frame of the observer.

The speed of light in vacuum, c, together with the hyperfine transition frequency of the caesium 133 atom, ΔνCs, forms the basis for the definition of the metre.