What is the exact speed of light in a vacuum, a value central to electromagnetic theory?

The exact speed of light in a vacuum is 299,792,458 metres per second.

This value is exact because the metre has been defined since 1983 as the distance light travels in vacuum during 1/299,792,458 of a second. Before that definition, scientists measured the speed experimentally with increasingly precise optical and astronomical methods.

James Clerk Maxwell’s equations predicted that electromagnetic waves travel at a speed determined by electrical permittivity and magnetic permeability. This helped identify light as electromagnetic radiation. The speed is usually written as c and applies to vacuum; light travels more slowly through materials such as glass or water.

The rounded figure of 300 million metres per second is useful for estimation, but it is not the exact value. The speed also underpins the modern definitions of the metre and the light-year.