View source for Metre

{{Short description|SI unit of length}}
{{About|the unit of length|other uses of "metre" or "meter"|Meter (disambiguation)}}
{{Use British English|date=September 2012}}
{{Use dmy dates|date=April 2020}}
{{Infobox unit
| name          = metre
| image         = Metric standards Rijksmuseum.jpg
| caption       = Historical replicas of metric standards, including an iron metre
| standard      = [[SI]]
| quantity      = [[length]]
| symbol        = m<ref>
  {{cite web
  | url=http://physics.nist.gov/cuu/Units/meter.html
  | title=Base unit definitions: Meter
  | publisher=[[National Institute of Standards and Technology]]
  | access-date=2010-09-28
  }}</ref>
| units1        = [[imperial units|Imperial]]/[[US customary units|US]] units
| inunits1      = {{unbulleted list
  | ≈&thinsp;{{val|1.0936|u=[[Yard|yd]]}}
  | ≈&thinsp;{{val|3.2808|u=[[Foot (unit)|ft]]}}
  | ≈&thinsp;{{val|39.37|u=[[Inch|in]]}}
  }}
| units2        = Nautical units
| inunits2      = ≈&thinsp;{{val|0.00053996|u=[[nautical mile|nmi]]}}
}}

The '''metre''' (or '''meter''' in [[American and British English spelling differences#-re, -er|US spelling]]; symbol: '''m''') is the [[SI base unit|base unit]] of [[length]] in the [[International System of Units]] (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of {{sfrac|{{val|299792458}}}} of a [[second]], where the second is defined by a [[Caesium standard|hyperfine transition frequency of caesium]].<ref name="SIBrochure9thEd">
{{citation
 |author=International Bureau of Weights and Measures
 |author-link=International Bureau of Weights and Measures
 |title=The International System of Units (SI)
 |date=20 May 2019
 |url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf
 |archive-url=https://web.archive.org/web/20211018184555/https://www.bipm.org/documents/20126/41483022/SI-Brochure-9.pdf/fcf090b2-04e6-88cc-1149-c3e029ad8232
 |url-status=live
 |edition=9th
 |isbn=978-92-822-2272-0
 |archive-date=18 October 2021
}}</ref>

The metre was originally defined in 1791 by the [[National Constituent Assembly (France)|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|Earth's polar circumference]] is approximately {{val|40,000|u=km}}.

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 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 vacuum in {{sfrac|1|{{val|299792458}}}} of a [[second]]. After the [[2019 revision of the SI]], this definition was rephrased to include the definition of a second in terms of the caesium frequency {{math|Δ''&nu;''<sub>Cs</sub>}}. This series of amendments did not alter the size of the metre significantly – today Earth's polar circumference measures {{val|40007.863|u=km}}, a change of about 200 [[parts per million]] from the original value of exactly {{val|40,000|u=km}}, which also includes improvements in the accuracy of measuring the circumference.

== Spelling ==
''Metre'' is the standard spelling of the metric unit for length in nearly all English-speaking nations, the exceptions being the United States<ref>
{{cite web
 |url=https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.330-2019.pdf
 |title=The International System of Units (SI) – NIST
 |publisher=[[National Institute of Standards and Technology]]
 |location=US
 |date=26 March 2008
 |quote=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.
}}</ref><ref>The most recent official brochure about the International System of Units (SI), written in French by the ''{{lang|fr|Bureau international des poids et mesures}}'', [[International Bureau of Weights and Measures]] (BIPM) uses the spelling ''metre''; an English translation, included to make the SI standard more widely accessible also uses the spelling ''metre'' ([[#bipm2006|BIPM, 2006]], p. 130''ff''). However, in 2008 the U.S. English translation published by the U.S. [[National Institute of Standards and Technology]] (NIST) chose to use the spelling ''meter'' 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|Turner]]). In 2008, NIST published the US version ([[#taylor2008a|Taylor and Thompson, 2008a]]) of the English text of the eighth edition of the BIPM publication ''{{lang|fr|Le 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 ([[#taylor2008a|Taylor and Thompson (2008a), p. iii]]). The Director of the NIST officially recognised this publication, together with [[#taylor2008b|Taylor and Thompson (2008b)]], as the "legal interpretation" of the SI for the United States ([[#turner|Turner]]). Thus, the spelling ''metre'' is referred to as the "international spelling"; the spelling ''meter'', as the "American spelling".</ref><ref>
{{cite web
 |url=http://www.metricationmatters.com/docs/Spelling_metre_or_meter.pdf
 |title=Spelling metre or meter
 |first=Pat |last=Naughtin
 |website=Metrication Matters
 |year=2008
 |access-date=2017-03-12
 |archive-url=https://web.archive.org/web/20161011100154/http://www.metricationmatters.com/docs/Spelling_metre_or_meter.pdf
 |archive-date=11 October 2016|url-status=usurped}}</ref><ref>{{cite web |url= http://grammarist.com/spelling/meter-metre/
 |title=Meter vs. metre
 |website=Grammarist
 |date=21 February 2011
 |access-date=2017-03-12
}}</ref> and the Philippines<ref>The Philippines uses [[Philippine English|English]] 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 the [[metric system]] uses ''metre'' ([[#PH-BatasPambansa8|Batas Pambansa Blg. 8]]) following the SI spelling, in actual practice, ''meter'' is used in government and everyday commerce, as evidenced by laws (''kilometer'', [[#PH-RA7160|Republic Act No. 7160]]), Supreme Court decisions (''meter'',  [[#PH-GR185240|G.R. No. 185240]]), and national standards (''centimeter'', [[#PH-PNSBAFS181-2016|PNS/BAFS 181:2016]]).</ref> which use ''meter''.

Measuring devices (such as [[ammeter]], [[speedometer]]) are spelled "-meter" in all variants of English.<ref>{{cite encyclopedia
 |url=http://dictionary.cambridge.org/results.asp?searchword=ammeter
 |archive-url=https://archive.today/20130703210011/http://dictionary.cambridge.org/results.asp?searchword=ammeter
 |url-status=dead
 |archive-date=3 July 2013
 |title=Cambridge Advanced Learner's Dictionary
 |year=2008
 |publisher=[[Cambridge University Press]]
 |access-date=2012-09-19
 }}, s.v. ammeter, meter, parking meter, speedometer.</ref> The suffix "-meter" has the same Greek origin as the unit of length.<ref>
{{cite encyclopedia
 |title=American Heritage Dictionary of the English Language
 |edition=3rd
 |year=1992
 |location=Boston
 |publisher=[[Houghton Mifflin]]
}}, s.v. meter.</ref><ref>
{{cite web
 |title=-meter – definition of -meter in English
 |url=https://en.oxforddictionaries.com/definition/-meter
 |archive-url=https://web.archive.org/web/20170426153254/https://en.oxforddictionaries.com/definition/-meter
 |url-status=dead
 |archive-date=26 April 2017
 |publisher=Oxford Dictionaries
}}</ref>

== Etymology ==
The etymological roots of ''metre'' can be traced to the Greek verb {{lang|grc|μετρέω}} ({{transliteration|grc|metreo}}) ((I) measure, count or compare)<ref>{{LSJ|metre/w|μετρέω|ref}}.</ref> and noun {{lang|grc|μέτρον}} ({{transliteration|grc|metron}}) (a measure),<ref>{{LSJ|me/tron|μέτρον|shortref}}.</ref> 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 ({{lang|la|metior, mensura}}), French ({{lang|fr|mètre, mesure}}), English and other languages. The Greek word is derived from the Proto-Indo-European root ''[[wikt:*meh₁-|*meh₁-]]'' 'to measure'. The motto {{lang|grc|ΜΕΤΡΩ ΧΡΩ}} ({{transliteration|grc|metro chro}}) on the seal of the [[International Bureau of Weights and Measures]] (BIPM) was approved by [[Adolphe Hirsch]] on 11 July 1875 and may be translated as "Keep the measure"; it thus calls for both measurement and moderation.<ref name=":6">{{Cite web |title=History – The BIPM 150 |url=https://thebipm150.org/history/ |access-date=2025-01-24 |language=}}</ref> In English, the use of the word ''metre'' (for the French unit {{lang|fr|mètre}}) began at least as early as 1797.<ref name="Oxford">[[Oxford English Dictionary]], Clarendon Press 2nd ed. 1989, vol. IX p. 697 col. 3.</ref>

== History of definition ==
{{excerpt|History of the metre}}

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.”<ref>{{cite web |title=World Metrology Day 2025 |url=https://www.worldmetrologyday.org/ |website=World Metrology Day |publisher=BIPM and OIML |access-date=3 September 2025}}</ref><ref>{{cite press release |title=Metre Convention events in Paris and Versailles |url=https://www.bipm.org/en/-/2025-06-18-metre-convention-events |website=BIPM |access-date=3 September 2025}}</ref>

== SI prefixed forms of metre ==
{{Main|Orders of magnitude (length)}}
[[SI prefix]]es 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 unit]]s (149.6&nbsp;Gm), [[light-year]]s (10&nbsp;Pm), or [[parsec]]s (31&nbsp;Pm), rather than in Mm or larger multiples; "30&nbsp;cm", "30&nbsp;m", and "300&nbsp;m" are more common than "3&nbsp;dm", "3&nbsp;dam", and "3&nbsp;hm", respectively.

The terms ''[[micron]]'' and ''[[millimicron]]'' have been used instead of ''micrometre'' (μm) and ''nanometre'' (nm), respectively, but this practice is discouraged.<ref>[[#taylor2008b|Taylor & Thompson 2003, p. 11.]]</ref>

{{SI multiples
| unit=metre
| symbol=m
| xd=[[decimetre]]
| xda=[[decametre]]
| xc=[[centimetre]]
| xh=[[hectometre]]
| xm=[[millimetre]]
| xk=[[kilometre]]
| xmc=[[micrometre]]
| xn=[[nanometre]]
| xp=[[picometre]]
| xf=[[femtometre]]
}}

== Equivalents in other units ==
{| class=wikitable style="margin:0 auto;"
|-
! colspan="5" style="text-align:left;"|Metric unit<br/>expressed in non-SI units
! colspan="4" style="text-align:left;"|Non-SI unit<br/>expressed in metric units
|-
| 1&nbsp;metre ||≈ ||style="text-align:right;"|1.0936 ||[[yard]]||
| 1&nbsp;yard||= ||style="text-align:right;"|0.9144 ||metre
|-
| 1&nbsp;metre ||≈ ||style="text-align:right;"|39.370 ||[[inch]]es||
| 1&nbsp;inch||= ||style="text-align:right;"|0.0254 ||metre
|-
| 1&nbsp;[[centimetre]] ||≈ ||style="text-align:right;"|{{val|0.39370}} ||inch||
| 1&nbsp;inch||= ||style="text-align:right;"|2.54 ||centimetres
|-
| 1&nbsp;[[millimetre]] ||≈ ||style="text-align:right;"|{{val|0.039370}} ||inch||
| 1&nbsp;inch||= ||style="text-align:right;"|25.4 ||millimetres
|-
| 1&nbsp;metre ||= ||style="text-align:right;"|10{{sup|10}}||[[ångström]]||
| 1&nbsp;ångström||= ||style="text-align:right;"|10{{sup|−10}} ||metre
|-
| 1&nbsp;[[nanometre]] ||= ||style="text-align:right;"|10||ångström||
| 1&nbsp;ångström||= ||style="text-align:right;"|100 ||[[picometre]]s
|}
Within this table, "inch" and "yard" mean "international inch" and "international yard"<ref>[[#AstinKaro1959|Astin & Karo 1959]].</ref> 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 to {{sfrac|5 000|127}}&nbsp;inches and to {{sfrac|1 250|1 143}}&nbsp;yards.
<!-- 1 metre ≈ 39.370 078 740 157 5 in / or 39.375 in = 1000.125 mm // what is this for? -->

A simple [[mnemonic]] to assist with conversion is "three 3s": 1&nbsp;metre is nearly equivalent to 3&nbsp;[[Foot (unit)|feet]] {{frac|3|3|8}}&nbsp;inches. This gives an overestimate of 0.125&nbsp;mm.

The ancient Egyptian [[cubit]] was about 0.5&nbsp;m (surviving rods are 523–529&nbsp;mm).<ref>Arnold Dieter (1991). [https://books.google.com/books?id=DU04vCP_TFAC ''Building in Egypt: pharaonic stone masonry'']. Oxford: Oxford University Press. {{ISBN|978-0-19-506350-9}}. p.251.</ref> Scottish and English definitions of the [[ell]] (2&nbsp;cubits) were 941&nbsp;mm (0.941&nbsp;m) and 1143&nbsp;mm (1.143&nbsp;m) respectively.<ref>{{Cite web |url=http://www.dsl.ac.uk/getent4.php?plen=7441&startset=10747969&query=ELL&fhit=ell&dregion=form&dtext=snd#fhit |title=Dictionary of the Scots Language |access-date=2011-08-06 |archive-url=https://web.archive.org/web/20120321184808/http://www.dsl.ac.uk/getent4.php?plen=7441&startset=10747969&query=ELL&fhit=ell&dregion=form&dtext=snd#fhit |archive-date=2012-03-21 |url-status=dead }}</ref><ref>{{Cite book |url=https://books.google.com/books?id=-BHnAAAAMAAJ&pg=PA221 |title=The Penny Magazine of the Society for the Diffusion of Useful Knowledge |publisher=Charles Knight |pages=221–22 |date=1840-06-06 |df=dmy-all}}</ref> The ancient Parisian ''toise'' (fathom) was slightly shorter than 2&nbsp;m and was standardised at exactly 2&nbsp;m in the [[mesures usuelles]] system, such that 1&nbsp;m was exactly {{frac|1|2}}&nbsp;toise.<ref name=H&H>{{cite web |url = https://archive.org/details/outlinesofevolut00halluoft/page/66 |title = Outlines of the evolution of weights and measures and the metric system |first1 = William  |last1 = Hallock |first2 = Herbert T |last2 = Wade |publisher = The Macmillan Company |year = 1906 |pages = 66–69|location = London}}</ref> The Russian [[verst]] was 1.0668&nbsp;km.{{sfn|Cardarelli|2004}} The [[Scandinavian mile|Swedish mil]] was 10.688&nbsp;km, but was changed to 10&nbsp;km when Sweden converted to metric units.<ref>{{cite encyclopedia |url=https://snl.no/mil |title=Mil |encyclopedia=Store norske leksikon |first=Knut |last=Hofstad |access-date=2019-10-18 |df=dmy-all}}</ref>

== See also ==
{{Commons category}}
{{Wiktionary|metre}}
* [[ISO 1]]{{spaced ndash}}standard reference temperature for length measurements
* [[Metric prefix]]
* [[Metrication]]

== References ==
{{Reflist|30em}}

=== Cited bibliography ===
{{refbegin}}
* {{Anchor|Alder2002}}{{cite book |title=The Measure of All Things : The Seven-Year Odyssey and Hidden Error That Transformed the World |first=Ken |last=Alder |year=2002 |publisher=Free Press |location=New York |isbn=978-0-7432-1675-3 |url-access=registration |url=https://archive.org/details/measureofallthin00alde }}
* {{Anchor|AstinKaro1959}}Astin, A. V. & Karo, H. Arnold, (1959), [http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf ''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)
* {{Anchor|BarbrowJudson1976}}{{cite book | title=Weights and Measures Standards of the United States, a brief history | first=Lewis V. | last=Judson | others=Derived from a prior work by Louis A. Fisher (1905) | editor-first=Louis E. | editor-last=Barbrow | publisher=[[US Department of Commerce]], [[National Bureau of Standards]] | location=US | date=1976-10-01 | orig-year=1963<!-- 1963-03 --> | id=NBS Special Publication 447; NIST SP 447; 003-003-01654-3 | lccn=76-600055 | doi=10.6028/NBS.SP.447}}
* {{Cite book |last=Bigourdan |first=Guillaume |url=https://archive.org/details/lesystmemtri00bigo|title=Le système métrique des poids et mesures; son établissement et sa propagation graduelle, avec l'histoire des opérations qui ont servi à déterminer le mètre et le kilogramme |trans-title=The metric system of weights and measures; its establishment and gradual propagation, with the history of the operations which served to determine the meter and the kilogram |date=1901 |publisher=Gauthier-Villars |location=Paris}}
* {{cite EB1911|first1=Alexander Ross |last1=Clarke |author1-link=Alexander Ross Clarke |first2=Friedrich Robert |last2=Helmert |author2-link=Friedrich Robert Helmert |date=1911b |wstitle=Earth, Figure of the|volume=8|pages=801–813}}
* {{Anchor|Guedj2001}}{{cite book |title=La Mesure du Monde |trans-title=The Measure of the World |first=Denis |last=Guedj |translator-first=Art |translator-last=Goldhammer |publisher=University of Chicago Press |location=Chicago |year=2001}}
* {{Anchor|Cardarelli2003}}{{cite book |title=Encydopaedia of scientific units, weights, and measures: their SI equivalences and origins |last=Cardarelli |first=François |year=2003 |publisher=Springer-Verlag London Limited |isbn=978-1-85233-682-0 |chapter=Chapter 2: The International system of Units |chapter-url=http://www.francoiscardarelli.ca/PDF_Files/ESU_Sample_Chapter_Section_1_2.pdf#page=6 |at=Table 2.1, p. 5 |quote=Data from Giacomo, P., Du platine à la lumière [From platinum to light], ''Bull. Bur. Nat. Metrologie'', '''102''' (1995) 5–14. |access-date=2017-01-26 |url=https://archive.org/details/encyclopaediaofs0000card }}
* {{cite book |last=Cardarelli |first=F. |year=2004 |title=Encyclopaedia of Scientific Units, Weights and Measures: Their SI Equivalences and Origins |publisher=Springer |edition=2nd |pages=[https://archive.org/details/encyclopaediaofs0000card/page/120 120]–124 |isbn=1-85233-682-X |url=https://archive.org/details/encyclopaediaofs0000card |url-access=registration}}
* {{anchor|nistmetre}}[http://physics.nist.gov/cuu/Units/meter.html ''Historical context of the SI: Meter'']. Retrieved 26 May 2010.
* {{Anchor|NIST2011}}National Institute of Standards and Technology. (27 June 2011). ''[https://www.nist.gov/pml/div688/grp50/primary-frequency-standards.cfm NIST-F1 Cesium Fountain Atomic Clock]''. Author.
* {{Anchor|NPL2010}}National Physical Laboratory. (25 March 2010). ''[http://www.npl.co.uk/science-technology/time-frequency/optical-frequency-standards-and-metrology/research/iodine-stabilised-lasers Iodine-Stabilised Lasers]''. Author.
* {{Anchor|NRC2010}}{{cite web|url=http://www.nrc-cnrc.gc.ca/eng/projects/inms/si-length.html |publisher=National Research Council Canada |title=Maintaining the SI unit of length |date=5 February 2010 |archive-url=https://web.archive.org/web/20111204014454/http://www.nrc-cnrc.gc.ca/eng/projects/inms/si-length.html |archive-date=4 December 2011 |url-status=dead }}
* {{Anchor|PH-BatasPambansa8}}Republic of the Philippines. (2 December 1978). ''[http://www.chanrobles.com/bataspambansabilang8.htm Batas Pambansa Blg. 8: An Act Defining the Metric System and its Units, Providing for its Implementation and for Other Purposes]''. Author.
* {{Anchor|PH-RA7160}}Republic of the Philippines. (10 October 1991). ''[https://www.officialgazette.gov.ph/downloads/1991/10oct/19911010-RA-7160-CCA.pdf Republic Act No. 7160: The Local Government Code of the Philippines]''. Author.
* {{Anchor|PH-GR185240}} Supreme Court of the Philippines (Second Division). (20 January 2010). ''[https://web.archive.org/web/20180427202441/http://sc.judiciary.gov.ph/jurisprudence/2010/january2010/185240.htm G.R. No. 185240]''. Author.
* {{Anchor|taylor2008a}}Taylor, B.N. and Thompson, A. (Eds.). (2008a). [https://web.archive.org/web/20171120061639/https://www.nist.gov/sites/default/files/documents/2016/12/07/sp330.pdf ''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 publication ''Le Système International d' Unités (SI)'' (Special Publication 330). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 18 August 2008.
* {{Anchor|taylor2008b}}Taylor, B.N. and Thompson, A. (2008b). [http://physics.nist.gov/cuu/pdf/sp811.pdf ''Guide for the Use of the International System of Units''] (Special Publication 811). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 23 August 2008.
* {{Anchor|turner}}Turner, J. (deputy director of the National Institute of Standards and Technology). (16 May 2008). [https://www.nist.gov/pml/wmd/metric/upload/FRN_Vol_73_No_96_16May2008_SI_Interpretation.pdf "Interpretation of the International System of Units (the Metric System of Measurement) for the United States"]. ''Federal Register'' Vol. 73, No. 96, p.{{nbsp}}28432–28433.
* {{Anchor|Zagar1999}}Zagar, B.G. (1999). [https://books.google.com/books?id=VXQdq0B3tnUC&pg=PT164#PPT160,M1 Laser interferometer displacement sensors] in J.G. Webster (ed.). ''The Measurement, Instrumentation, and Sensors Handbook''. CRC Press. {{ISBN|0-8493-8347-1}}.
{{refend}}

{{SI units}}
{{SI units of length}}
{{Authority control}}

[[Category:Metre| ]]
[[Category:SI base units]]
[[Category:Units of length]]
[[Category:1790s introductions]]