# Speed of light conversions

The speed of light conversion selector above selects the speed/velocity measurement unit to convert to starting from speed of light (c). To make a conversion starting from a unit of speed other than speed of light, simply click on the "Reset" button.

## About speed of light

The **speed of light** is a unit of speed equal to one Plank unit or 299,792,458 meters per second (1 c = 299,792,458 m/s), the derived unit of speed in the International System of Units (SI).

The speed of light (c) is also equal to approximately 1,080 million kilometers per hour (km/h), or about 300,000 kilometers per second (km/s), or nearly 671 million miles per hour (mph), or 186,282 miles per second (mps or mi/s) or 173.144 astronomical units per day (AU/day).

The speed of light (c) is defined as the maximum speed at which all matter, information, and energy in the universe can travel in vacuum.

In a medium, light travels at a speed (v) less than the speed of light (c), depending on the refractive index (n) of the medium.

v = c/n

For instance, for visible light:

- The speed light travels through air is equal to about c/1.0003 ≈ 299,702.547 km/s (i.e., about 90 km/s slower than c)
- The speed light travels through water is equal to about c/1.3 ≈ 230,609.583 km/s
- The speed light travels through glass is equal to about c/1.5 ≈ 199,861.639 km/s
- The speed light travels through diamond is equal to about c/2.4 ≈ 124,913.524 km/s

Symbol: c

Plural: speed of light