Metre

metre
Seal of the International Bureau of Weights and Measures (BIPM) – Use measure (Greek: ΜΕΤΡΩ ΧΡΩ)
General information
Unit system	SI
Unit of	length
Symbol	m[1]
Conversions
1 m[1] in ...	... is equal to ...
SI units	1000 mm 0.001 km
Imperial/US units	≈ 1.0936 yd ≈ 3.2808 ft ≈ 39.37 in
Nautical units	≈ 0.00053996 nmi

The metre (or meter in American spelling; symbol: m) is the base unit of length in the International System of Units (SI).

The metre was originally defined in 1791 by the French National Assembly as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's circumference is approximately 40000 km. In 1799, the metre was redefined in terms of a prototype metre bar. The actual bar used was changed in 1889. In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86.

The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the metre is a measure of proper length. From 1983 until 2019, the metre was formally defined as the length of the path travelled by light in a vacuum in 1/299792458 of a second. After the 2019 redefinition of the SI base units, this definition was rephrased to include the definition of a second in terms of the caesium frequency Δν_Cs.