Speed of light in vacuum

speedOfLight, c, is the exact speed at which light propagates in vacuum, expressed in metres per second. It defines the metre and connects space, time, mass, and electromagnetic waves.

Practical use

Use c to convert wavelength and frequency or mass and rest energy. Pair it with planckConstant for photon energy and planckTime for natural units.

Caveats

Light travels more slowly in material media. Do not confuse c with a signal speed in every system; use divide with explicit units and pow for c² in E = mc².

Use Speed Of Light in a calculation

Enter speedOfLight on a calculator line to see its built-in value, or place the name inside a longer expression. Keep any displayed unit attached when converting or combining physical quantities, and check that every other value uses a compatible dimension.

The example above is editable, so you can replace surrounding values and test the constant in your own formula. Browse all constants to compare related values and notation.