Metre

The metre (symbol m) is the SI base unit of length. It is used to measure length and distance in science, engineering, and everyday life. The current definition fixes the metre as the distance travelled by light in vacuum in 1/299,792,458 of a second, making the speed of light exactly 299,792,458 metres per second.

Historically, the metre was first defined in 1799 as one ten-millionth of the distance from the equator to the North Pole along a meridian. In 1889 the International Prototype of the Mètre (IPM), a platinum-iridium bar, was adopted as the standard and kept at the International Bureau of Weights and Measures (BIPM) in Sèvres, France. In 1960 the metre was redefined in terms of the wavelength of light emitted by krypton-86. The 1983 redefinition then fixed the metre to the distance light travels in vacuum during 1/299,792,458 of a second.

The metre is the basis for other length units, such as the kilometre (1000 m), centimetre (0.01 m), millimetre (0.001 m), micrometre (1e-6 m), and nanometre (1e-9 m). SI prefixes apply to metres for a wide range of lengths. Measurements are traceable to international standards through national metrology institutes.

The metre is defined and maintained by the BIPM and national institutes. Real-world length measurements use calibrated instruments and standards to ensure accuracy, such as rulers, tapes, interferometers, and coordinate-measuring machines. In science and industry, the metre provides a common reference for quantitative comparisons.