# Speed Of Light Constant

Use Speed Of Light in Calcul.io. See what this built-in constant represents, enter it in an editable calculation, and review related values and units.

## speedOfLight

Speed of light in vacuum

Try it yourself:

```calculio
speedOfLight
```

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## 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](https://calcul.io/constants/planckConstant/index.md) for photon energy and [planckTime](https://calcul.io/constants/planckTime/index.md) for natural units.

## Caveats

Light travels more slowly in material media. Do not confuse c with a signal speed in every system; use [divide](https://calcul.io/function/divide/index.md) with explicit units and [pow](https://calcul.io/function/pow/index.md) for c² in E = mc².

[All constants](https://calcul.io/constants/index.md)