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Speed of light

The defined speed of light in vacuum is exactly 299,792,458 m/s. NIST, the BIPM and the Wikipedia constant table all print that exact value.

Also known as c, speed of light in vacuum
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Velocity

Published as 299,792,458 m/sPublished as 299,792,458 m/s

Exact defined value of the speed of light in vacuum. The metre is defined from this number. Not a measured uncertainty.

OfficialBIPM (opens source in a new tab)

In context

  • 26,931 laps of Earth an hourConstant · Earth's equator, 40,075 km (WGS 84)
  • London to New York in 0.0 sConstant · London to New York, 5,570 km great-circle
  • 10,060× Earth's speed around the SunSourced · Earth, 29.8 km/s. NASA planetary fact sheet, mean orbital speed

Worked out from the sourced figure above, which stays the ranked number.

Mechanism

Why it's so fast

The speed of light in vacuum is exact because the metre is defined from it. Since 1983, one metre is the distance light travels in a fixed fraction of a second. The number cannot come out a little high. Nothing with mass reaches it.

  1. A definition, not a race

    The BIPM states that the speed of light in vacuum is exactly 299,792,458 metres per second, because that is how the metre is fixed. NIST prints the same exact constant. A rounded miles-per-second figure is a reminder, not a second experiment.

  2. Mass cannot catch it

    Adding speed to an object with mass takes more and more energy as the speed gets closer to this one. The rankings on this site, from a cheetah to a solar probe, sit far below it. Percent of light speed is a comparison beside the sourced figure, not a replacement.

  3. Vacuum, not a cable

    Light in air, glass or water is slower. The exact number is the vacuum speed. A network ping is not a measurement of it, and a game that says light speed has not changed the metre.

The pressure wave in air is the speed of sound. A spacecraft that is merely very fast is Parker Solar Probe.

Provenance

Every measurement

Claimed, measured and official figures side by side, each with its source. The ranked figure is marked.

  1. 299,792.5km/s670,616,630mphRanked figure

    Exact defined value of the speed of light in vacuum. The metre is defined from this number. Not a measured uncertainty.

    Official
    BIPM (opens source in a new tab)

Reference

Specs

Symbol
c
Medium
Vacuum
Value
299,792,458 m/s
Status
Exact by definition

Context

Background

The BIPM's definition of the metre takes the fixed numerical value of the speed of light in vacuum to be 299,792,458 when expressed in metres per second, and says that implies the exact relation c = 299,792,458 m/s. NIST's 2022 CODATA table prints 299,792,458 m/s with the uncertainty marked exact. Wikipedia's constant table prints the same metres-per-second figure as an exact value.

The row is a reference, not a race. The speed of sound in air at 20°C is its own row, at the textbook 343 m/s. A rounded 186,000 miles per second is this constant written coarsely, and it is not stored.

Spot a wrong number?

Every report is read by a person, and corrections are dated on the page.