Metre (meter): the SI base unit of length

Modern definition and realization

Since 1983 the speed of light in vacuum has been fixed by definition, and the metre is defined as the distance light travels in vacuum in 1/299,792,458 of a second. This definition ties the unit of length to the unit of time and to a universal physical constant, allowing laboratories to realise the metre with extremely high precision using optical frequencies and atomic clocks. Practical realisations use interferometry and stabilized lasers referenced to frequency standards.

Historical development

The idea of a universal length unit arose during the French Revolution, when scientists sought a natural and reproducible standard. The original proposal defined the metre as one ten-millionth of the distance between the Equator and the North Pole measured along the meridian through Paris. That geodetic concept depended on measuring a distance on the surface of the Earth along the Paris meridian. Later practical standards included a platinum-iridium prototype bar and an optical wavelength standard before the present definition based on light.

Metric multiples and common sizes

The SI prefixes make the metre convenient across many scales: millimetre (mm), centimetre (cm), kilometre (km) and beyond. Typical references help visualise the unit: an adult human height is roughly 1.7 m, an Olympic swimming pool is 50 m, and road distances are commonly given in kilometres (1 km = 1,000 m). Conversions to some imperial units are widely used for comparison: one yard is exactly 0.9144 metre by international agreement, so a metre equals about 39.37 inches or approximately 3.281 feet.

Applications and significance

The metre underpins measurements in physics, astronomy, engineering, surveying and construction. Its modern definition, tied to a fundamental constant and to the second, ensures traceability and reproducibility across national metrology institutes. That link between time and length enables extremely precise measurements needed for technologies such as global navigation, telecommunications and high-resolution manufacturing.

Further information

Readers seeking more technical or historical detail can consult international standards and metrology organisations for official documents and guides. Introductory educational materials explain practical measurement techniques and the use of SI prefixes, while specialised literature covers the experimental methods used to realise the metre in national laboratories. For concise summaries and official definitions see the entries provided by standards bodies and scientific organizations referenced here: distance, Earth, meridian, and additional resources on the nature of light and vacuum (light, vacuum).