Speed of light in vacuum, c

The speed of light, and of every other electromagnetic wave, in a vacuum. It has been fixed by definition since 1983, so the value below is not a measurement and carries no uncertainty.

c = 299,792,458 m/s

Exact by definition, so it has no uncertainty at all.

In other units

Values marked … are exact, but their decimals run on, so they stop at ten significant figures instead of being rounded.

Speed of light in vacuum in SI units and in other units
Quantity	Value
c	299,792,458 m/s
c	299,792.458 km/s
c	1,079,252,848.8 km/h
c	670,616,629.3… mph
c	983,571,056.4… ft/s
c	0.299792458 m per nanosecond

c in km/s

299,792,458 m/s ÷ 1000 m/km
= 299,792.458 km/s

c in km/h

299,792,458 m/s × 3600 s/h ÷ 1000 m/km
= 1,079,252,848.8 km/h

c in mph

299,792,458 m/s × 3600 s/h ÷ 1609.344 m/mile
= 670,616,629.3… mph

c in ft/s

299,792,458 m/s ÷ 0.3048 m/ft
= 983,571,056.4… ft/s

c in m per nanosecond

299,792,458 m/s × 10⁻⁹ s
= 0.299792458 m per nanosecond

Just under 30 cm in a nanosecond, which is also the vacuum wavelength of a 1 GHz signal.

Where the value comes from

The figure 299,792,458 m/s is not a measurement result. In 1983 the 17th General Conference on Weights and Measures redefined the metre as the distance light travels in vacuum in 1/299,792,458 of a second, which made the speed of light a fixed number and the metre a consequence of it and of the second.

The number was chosen to agree with the best measurements of the day, so no ruler changed length. What changed was the direction of the dependency: a sharper experiment now improves how well a laboratory can realise the metre, and c stays exactly where it is.

The 2019 definitions lean on it as well. The kilogram is defined through the Planck constant, whose unit, the joule-second, contains the metre, so the kilogram depends on c too.

Which form to use

For most calculations 2.998 × 10⁸ m/s is plenty: it is 0.0025 percent high. The textbook 3 × 10⁸ m/s is 0.069 percent high, finer than most school data but not good enough to check a result to five figures.

It appears across physics. It is the wave speed in c = fλ for every electromagnetic wave, it turns a photon’s wavelength into its energy in E = hc/λ, it relates mass and energy in E = mc², and it links the electric and magnetic constants through ε₀μ₀c² = 1.

In a material light is slower, by the refractive index: v = c/n. In water, with n = 1.333, that is 2.249 × 10⁸ m/s.

Common mistakes

Using c for light inside glass or water. c is the speed in a vacuum; in a medium the speed is c/n, and the wavelength shortens by the same factor while the frequency stays the same.
Reading the wavelength from c = fλ in the wrong unit. With c in m/s and f in hertz the wavelength comes out in metres, so visible light appears as something like 5 × 10⁻⁷ and still has to be multiplied by 10⁹ to read in nanometres.
Rounding c and then quoting an answer to more figures than the rounding allows. The 3 × 10⁸ m/s shortcut is only good to about 0.069 percent.

Tools that use it

Wavelength and Frequency Calculator takes it as the default wave speed in c = fλ.
Photon Energy Calculator uses hc to turn a wavelength into a photon energy.
Snell’s Law Calculator reports the speed of light in the second medium, c/n.
Refractive Index Table gives the speed of light in each material, c/n.
Electromagnetic Spectrum Explorer ties every wavelength on the spectrum to its frequency, c = λf.
Refraction Simulator shows the speed of light in each medium, c/n, beside the bending it causes.
Photoelectric Effect Simulator turns the wavelength of the light into its frequency, and the work function into a threshold wavelength, hc/φ.
Time Dilation Calculator divides a speed by it to give β = v/c, and from that the Lorentz factor γ.

Related constants

Planck constant, h = 6.62607015 × 10⁻³⁴ J·s
Vacuum permittivity, ε₀ = 8.8541878188(14) × 10⁻¹² F/m
Vacuum permeability, μ₀ = 1.25663706127(20) × 10⁻⁶ N/A²
Stefan-Boltzmann constant, σ = 5.670374419… × 10⁻⁸ W/(m²·K⁴)