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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →The speed of light in a vacuum, written c, is exactly 299,792,458 metres per second. In the International System of Units (SI) this is a fixed, defined value, not a measurement with an error bar. The metre is defined from it: one metre is the length of the path light travels in vacuum in 1/299,792,458 of a second.
The official definition
The International Bureau of Weights and Measures (BIPM) states that the SI unit is defined “by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency ΔνCs.” See the BIPM page SI base unit: metre.
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Written as an equation: c = 299,792,458 m/s. The number is exact in SI units because it is fixed by the definition itself (BIPM, SI defining constants).
How the value defines the metre
BIPM puts it this way: “The effect of this definition is that one metre is the length of the path travelled by light in vacuum during a time interval with duration of 1/299 792 458 of a second.”
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The definition chains together two things:
- The second is defined from the caesium-133 hyperfine transition frequency.
- The metre follows from that second and the fixed value of c.
Length is therefore derived from time. Nobody measures c in SI any more; instead, a better clock gives a better metre.
Vacuum only
The exact figure applies to light in vacuum. Light moves more slowly through materials such as glass or water, so 299,792,458 m/s should never be quoted as the speed of light in every medium. When the same speed is reported in other units, the vacuum value has not changed; only the unit has.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When the definition changed
| Date | Event | Source |
|---|---|---|
| 1983 | The 17th CGPM defined the metre as the distance light travels in vacuum in 1/299,792,458 of a second. | 17th CGPM, Resolution 1 |
| 20 May 2019 | The revised SI took effect, built on seven fixed defining constants, including c. The metre follows from c and the definition of the second. | 26th CGPM, Resolution 1 (2018) |
The 1983 wording defined the metre directly by light travel time. The 2019 revision kept the same numerical value but recast all unit definitions around fixed constants, so c now sits alongside the caesium frequency among the defining constants.
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