The speed of light
In vacuum, light travels exactly 299,792,458 metres per second. The word "exactly" is literal: since 1983 the metre is defined from this number, so the value carries no uncertainty at all.
Updated on October 7, 2026
The value, and the same value in other units
Every figure below is converted from the defining value by our own code, using exact unit definitions (the international mile is exactly 1,609.344 m and the astronomical unit exactly 149,597,870,700 m), so none of the conversions adds an error of its own.
c = 299,792,458 m s⁻¹
- Value
- 2.99792458 × 10⁸
- Unit
- m s⁻¹
- Standard uncertainty
- exact by definition (zero uncertainty)
The same value in other units
- m/s
- 299,792,458
- km/s
- 299,792.458
- km/h
- 1,079,252,848.8
- mph
- 670,616,629
- mi/s
- 186,282.397
- 1 light year (m)
- 9,460,730,472,580,800
- Terra → Lua
- 1 s
- Sol → Terra (1 ua)
- 8 min 19 s
- Proxima Centauri → Terra
- 4.247 a
Source: NIST — CODATA Internationally Recommended Values of the Fundamental Physical Constants (2022)
Why there is no uncertainty
Until 1983 the metre was a physical length and the speed of light was something you measured, with error bars that shrank decade by decade. Then the definition was turned around: the metre became the distance light travels in 1/299,792,458 of a second. From that moment the number stopped being a measurement and became a definition, which is why you will never see it quoted with a ± figure.
What is still measured is anything light travels through. In water light slows to about three quarters of c, in glass to roughly two thirds. The constant 299,792,458 m/s is specifically the vacuum value, and it is the speed that appears in E = mc² and in the cosmic speed limit.
Source: BIPM — Sistema Internacional de Unidades (SI), 9ª edição
How to read this page
How to read the numbers here
Every value on "The speed of light" comes from the source named beside its section, never from an estimate of our own with no origin.
Where the data comes from
Each section of "The speed of light" names its official source; the verification date sits at the top of the page.
What the number doesn’t tell you
Atomic weight, molar mass and standard uncertainty are measurement concepts: none of them say anything about a real sample’s purity, physical state or specific isotope.
More about “The speed of light”
c among the constants this site tracks
Of the 9 physical constants this module tracks, 7 are exact by definition since the 2019 SI revision — and c is one of them. Its value has order of magnitude 108 in the unit shown on "The speed of light".
The order of magnitude alone says a lot: a value around 108 needs scientific notation to avoid a row of zeros, which is exactly why physics favours that notation over loose decimals.
Words on this page
Standard uncertainty
The margin attached to a measured value, usually written in parentheses on the last digits. A constant that is "exact by definition" has zero standard uncertainty; a measured one, like the gravitational constant G, always carries some.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Standard uncertainty
Mole
The SI counting unit for atoms, molecules or other entities: one mole is exactly 6.02214076 × 10²³ of them, by the definition fixed in 2019. It is what ties a mass in grams to a number of particles.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Mole
Atomic weight
The average weight of an element’s atoms, weighted by how abundant each isotope is in nature, in atomic mass units. For 14 elements CIAAW publishes a range instead of a single value, because that abundance varies a little from sample to sample.
Standard definition by CIAAW — Commission on Isotopic Abundances and Atomic Weights · Atomic weight
Atomic number
The number of protons in an atom’s nucleus, written Z. It is this number, not the mass, that defines which element an atom is: every atom with 8 protons is oxygen, whether it has 8 or 18 neutrons.
Standard definition by IUPAC · Atomic number
Astronomical unit
The solar system’s reference distance, fixed at exactly 149,597,870,700 metres since 2012, close to the average Earth–Sun distance. It saves writing every planet’s orbit in kilometres with ten digits.
Standard definition by IAU — International Astronomical Union · Astronomical unit