What is the speed of light in a vacuum exactly?

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

This value is exact because the metre is defined using the distance light travels in a vacuum during a specified fraction of a second. Since 1983, the International System of Units has defined the metre as the distance light travels in 1/299,792,458 of a second, making the numerical value of c fixed by definition rather than measured with uncertainty.

The exact value is often rounded to 3.00 × 10⁸ m/s, 300,000 km/s or about 186,000 miles per second. Those forms are useful approximations, but they are not the exact SI answer. Light travels more slowly through materials such as air, water and glass because it interacts with the medium.

In special relativity, c is the invariant speed shared by light and other massless particles in vacuum. Massive objects can approach it but cannot reach it with finite energy. Light from the Sun takes about 8 minutes 20 seconds to reach Earth, illustrating that even this enormous speed is not instantaneous across space.