Definition · Plain-language
The metre
The metre is the SI base unit of length, defined by fixing the speed of light in a vacuum so that it can be realised anywhere, with no physical reference bar.
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The unit of length
The metre is the SI base unit for length, from which every other length unit in the system is built: the kilometre is a thousand metres, the centimetre a hundredth, the millimetre a thousandth. Its symbol is the lower-case m. As one of the seven base units, the metre also feeds into many derived units — area in square metres, volume in cubic metres, speed in metres per second. The spelling “metre” is standard in British English, while “meter” is the American form; both name the same unit. A metre is a little over a yard, roughly the height of a kitchen worktop.
Defined by the speed of light
Since 1983 the metre has been defined by fixing the speed of light in a vacuum at exactly 299,792,458 metres per second. The metre is then the distance light travels in a vacuum during 1 ⁄ 299,792,458 of a second. Because the second is itself defined by an atomic constant, length is tied to time and to an unchanging property of nature. This means the metre can be reproduced precisely in any properly equipped laboratory, without comparison to a physical object — a far more stable and universal standard than any manufactured bar could ever be.
From a platinum bar to a constant
The metre began in 1790s France as one ten-millionth of the distance from the equator to the North Pole, then for decades was embodied in a physical artefact — a platinum-iridium bar kept near Paris, with copies distributed worldwide. The trouble with any artefact is that it can be damaged or drift over time. The 1983 redefinition swept this away by anchoring the metre to the speed of light. The 2019 revision of the SI restated the definition in the same constant-based language now used for all the base units, but the practical length of the metre was unchanged.
Key facts
At a glance
- Definition: the SI base unit of length, symbol m
- Current basis: the distance light travels in 1 ⁄ 299,792,458 of a second
- Speed of light: fixed at exactly 299,792,458 m/s
- History: once a platinum-iridium bar near Paris (pre-1983)
- Spelling: “metre” in British English, “meter” in American
- Builds: kilometre, centimetre, millimetre and area/volume units
Common misconceptions
What people often get wrong
Often heard: The metre is still defined by a metal bar kept in Paris.
Actually: Not since 1983. The metre is now defined by fixing the speed of light, so it depends on a constant of nature rather than any physical object that could change.
Often heard: A metre and a yard are the same length.
Actually: They are close but not equal. One metre is about 1.094 yards, or roughly 39.37 inches — a little longer than a yard, which is 0.9144 metres exactly.
Often heard: The definition of the metre depends on the size of the Earth.
Actually: That was the original 1790s idea, but the modern metre is defined by the speed of light. Its length is now independent of any measurement of the Earth.
Going deeper
