In 1799, the metre was redefined in terms of a prototype metre bar. The bar used was changed in 1889, and in 1960 the metre was redefined in terms of a certain number of wavelengths of a certain emission line ofkrypton-86. The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the metre is a measure ofproper length. From 1983 until 2019, the metre was formally defined as the length of the path travelled bylight in vacuum in1/299792458 of asecond. After the2019 revision of the SI, this definition was rephrased to include the definition of a second in terms of the caesium frequencyΔνCs. This series of amendments did not alter the size of the metre significantly – today Earth's polar circumference measures40007.863 km, a change of about 200parts per million from the original value of exactly40000 km, which also includes improvements in the accuracy of measuring the circumference.
Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations, the exceptions being the United States[3][4][5][6] and the Philippines[7] which usemeter.
Measuring devices (such asammeter,speedometer) are spelled "-meter" in all variants of English.[8] The suffix "-meter" has the same Greek origin as the unit of length.[9][10]
The etymological roots ofmetre can be traced to the Greek verbμετρέω (metreo) ((I) measure, count or compare)[11] and nounμέτρον (metron) (a measure),[12] which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism (as in "be measured in your response"). This range of uses is also found in Latin (metior, mensura), French (mètre, mesure), English and other languages. The Greek word is derived from the Proto-Indo-European root*meh₁- 'to measure'. The mottoΜΕΤΡΩ ΧΡΩ (metro chro) on the seal of theInternational Bureau of Weights and Measures (BIPM) was approved byAdolphe Hirsch on 11 July 1875 and may be translated as "Keep the measure"; it thus calls for both measurement and moderation.[13] In English, the use of the wordmetre (for the French unitmètre) began at least as early as 1797.[14]
Replicas of historical metric standards, including an iron copy of themètre des Archives.
During theFrench Revolution, thetraditional units of measure were to be replaced by consistent measures based on natural phenomena. As a baseunit of length, scientists had favoured theseconds pendulum (a pendulum with a half-period of one second) one century earlier, but this was rejected as it had been discovered that this length varied from place to place with local gravity. Themètre was introduced – defined as one ten-millionth of the shortest distance from the North Pole to the equatorpassing through Paris, assuming anEarth flattening of1/334.[15]
Following thearc measurement of Delambre and Méchain, the historical French official standard of the metre was made available in the form of theMètre des Archives, aplatinum bar held in Paris. It was originally also planned to dematerialize the definition of the metre by counting the number of swings of a one-metre-long pendulum during a day at alatitude of 45°.[16] However, dematerializing the definition of units of length by means of the pendulum would prove less reliable than artefacts.[17][18]
TheMètre des Archives and its copies such as theCommittee Meter were replaced from 1889 at the initiative of theInternational Association of Geodesy by thirtyplatinum-iridium bars kept across the globe.[19] A betterstandardisation of the new prototypes of the metre and their comparison with each other and with the historical standard involved the development of specialised measuring equipment and the definition of a reproducible temperature scale.[20]
Progress in science finally allowed the definition of the metre to be dematerialised; thus in 1960 a new definition based on a specific number of wavelengths of light from a specific transition inkrypton-86 allowed the standard to be universally available by measurement. In 1983 this was updated to a length defined in terms of thespeed of light; this definition was reworded in 2019:[23]
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of thespeed of light in vacuumc to be299792458 when expressed in the unit m⋅s−1, where the second is defined in terms of the caesium frequencyΔνCs.
Where older traditional length measures are still used, they are now defined in terms of the metre – for example theyard has since 1959 officially been defined as exactly 0.9144 metre.[24]
In May 2025, the 150th anniversary of the Metre Convention was celebrated with events in Paris and Versailles, organized by the BIPM and the French government. The celebrations highlighted the long-standing international cooperation in metrology under the theme “Measurements for all times, for all people.”[25][26]
SI prefixes can be used to denote decimal multiples and submultiples of the metre, as shown in the table below. Long distances are usually expressed in km,astronomical units (149.6 Gm),light-years (10 Pm), orparsecs (31 Pm), rather than in Mm or larger multiples; "30 cm", "30 m", and "300 m" are more common than "3 dm", "3 dam", and "3 hm", respectively.
The termsmicron andmillimicron have been used instead ofmicrometre (μm) andnanometre (nm), respectively, but this practice is discouraged.[27]
Within this table, "inch" and "yard" mean "international inch" and "international yard"[28] respectively, though approximate conversions in the left column hold for both international and survey units.
"≈" means "is approximately equal to";
"=" means "is exactly equal to".
One metre is exactly equivalent to5 000/127 inches and to1 250/1 143 yards.
A simplemnemonic to assist with conversion is "three 3s": 1 metre is nearly equivalent to 3 feet3+3⁄8 inches. This gives an overestimate of 0.125 mm.
The ancient Egyptiancubit was about 0.5 m (surviving rods are 523–529 mm).[29] Scottish and English definitions of theell (2 cubits) were 941 mm (0.941 m) and 1143 mm (1.143 m) respectively.[30][31] The ancient Parisiantoise (fathom) was slightly shorter than 2 m and was standardised at exactly 2 m in themesures usuelles system, such that 1 m was exactly1⁄2 toise.[32] The Russianverst was 1.0668 km.[33] TheSwedish mil was 10.688 km, but was changed to 10 km when Sweden converted to metric units.[34]
^"The International System of Units (SI) – NIST"(PDF). US:National Institute of Standards and Technology. 26 March 2008.The spelling of English words is in accordance with the United States Government Printing Office Style Manual, which follows Webster's Third New International Dictionary rather than the Oxford Dictionary. Thus the spellings 'meter', 'liter', 'deka', and 'cesium' are used rather than 'metre', 'litre', 'deca', and 'caesium' as in the original BIPM English text.
^The most recent official brochure about the International System of Units (SI), written in French by theBureau international des poids et mesures,International Bureau of Weights and Measures (BIPM) uses the spellingmetre; an English translation, included to make the SI standard more widely accessible also uses the spellingmetre (BIPM, 2006, p. 130ff). However, in 2008 the U.S. English translation published by the U.S.National Institute of Standards and Technology (NIST) chose to use the spellingmeter in accordance with the United States Government Printing Office Style Manual. The Metric Conversion Act of 1975 gives the Secretary of Commerce of the US the responsibility of interpreting or modifying the SI for use in the US. The Secretary of Commerce delegated this authority to the Director of the National Institute of Standards and Technology (Turner). In 2008, NIST published the US version (Taylor and Thompson, 2008a) of the English text of the eighth edition of the BIPM publicationLe Système international d'unités (SI) (BIPM, 2006). In the NIST publication, the spellings "meter", "liter" and "deka" are used rather than "metre", "litre" and "deca" as in the original BIPM English text (Taylor and Thompson (2008a), p. iii). The Director of the NIST officially recognised this publication, together withTaylor and Thompson (2008b), as the "legal interpretation" of the SI for the United States (Turner). Thus, the spellingmetre is referred to as the "international spelling"; the spellingmeter, as the "American spelling".
^Naughtin, Pat (2008)."Spelling metre or meter"(PDF).Metrication Matters. Archived from the original on 11 October 2016. Retrieved12 March 2017.
^"Meter vs. metre".Grammarist. 21 February 2011. Retrieved12 March 2017.
^The Philippines usesEnglish as an official language and this largely follows American English since the country became a colony of the United States. While the law that converted the country to use themetric system usesmetre (Batas Pambansa Blg. 8) following the SI spelling, in actual practice,meter is used in government and everyday commerce, as evidenced by laws (kilometer,Republic Act No. 7160), Supreme Court decisions (meter,G.R. No. 185240), and national standards (centimeter,PNS/BAFS 181:2016).
^Quinn, T. J. (2012).From artefacts to atoms: the BIPM and the search for ultimate measurement standards. Oxford: Oxford University Press. pp. 12–14,8–12,49–72,106–108,144–147.ISBN978-0-19-990991-9.OCLC861693071.
Astin, A. V. & Karo, H. Arnold, (1959),Refinement of values for the yard and the pound, Washington DC: National Bureau of Standards, republished on National Geodetic Survey web site and the Federal Register (Doc. 59–5442, Filed, 30 June 1959)
Supreme Court of the Philippines (Second Division). (20 January 2010).G.R. No. 185240. Author.
Taylor, B.N. and Thompson, A. (Eds.). (2008a).The International System of Units (SI). United States version of the English text of the eighth edition (2006) of the International Bureau of Weights and Measures publicationLe Système International d' Unités (SI) (Special Publication 330). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 18 August 2008.
