What is the exact speed of light in vacuum in meters per second?

The exact speed of light in vacuum is 299,792,458 meters per second. The symbol c commonly represents this universal physical constant, and the value is exact rather than an experimental approximation.

The number became exact because the International System of Units defines the metre using the speed of light: one metre is the distance light travels in vacuum during a precisely specified fraction of a second. The second is tied to the frequency of radiation from cesium-133, so fixing c helps connect the units of time and length through a reproducible physical standard.

Light travels more slowly through materials such as air, water and glass because electromagnetic waves interact with matter. The quoted value applies specifically to a vacuum. In empty space, all electromagnetic radiation—including radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays—propagates at c.

According to special relativity, c is also the maximum speed for information and causal influence. Objects with nonzero rest mass can approach it but cannot be accelerated to exactly that speed. The rounded value 300,000,000 m/s is useful for estimates, but it is not the exact answer.