What exact speed of light in a vacuum is used in the International System of Units?

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

The value became exact in the International System of Units in 1983, when the metre was defined using the distance light travels in a specified fraction of a second. Because the metre is tied to this constant, the speed is no longer a measurement with experimental uncertainty in SI units.

Light travels more slowly through materials such as glass, water, and air. The vacuum value is the fundamental constant usually represented by the letter c. It is also the maximum speed at which information or matter can travel according to special relativity.

The approximate value of 300,000 kilometres per second is useful for mental calculations, but it is not the exact SI expression. Converting the exact value gives about 299,792 kilometres per second.