Metre

The metre (or meter in US spelling; symbol: m) is the 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 ⁠1/299792458⁠ of a second, where the second is defined by a hyperfine transition frequency of caesium.

metre
Historical public metre standard in Paris
General information
Unit system	SI
Unit of	length
Symbol	m
Conversions
1 min ...	... 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 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 polar circumference is approximately 40000 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 ⁠1/299792458⁠ 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 Δν_Cs. This series of amendments did not alter the size of the metre significantly – today Earth's polar circumference measures 40007.863 km, a change of about 200 parts per million from the original value of exactly 40000 km, which also includes improvements in the accuracy of measuring the circumference.

Spelling

Seal of the International Bureau of Weights and Measures (BIPM) – Use measure (Greek: ΜΕΤΡΩ ΧΡΩ)

Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations, the exceptions being the United States and the Philippines which use meter.

Measuring devices (such as ammeter, speedometer) are spelled "-meter" in all variants of English. The suffix "-meter" has the same Greek origin as the unit of length.

Etymology

The etymological roots of metre can be traced to the Greek verb μετρέω (metreo) ((I) measure, count or compare) and noun μέτρον (metron) (a measure), 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 motto ΜΕΤΡΩ ΧΡΩ (metro chro) in the seal of the International Bureau of Weights and Measures (BIPM), was approuved by Adolphe Hirsch on 11 July 1875 and may be translated as "Keep the measure", thus calls for both measurement and moderation. The use of the word metre (for the French unit mètre) in English began at least as early as 1797.

History of definition

This section is an excerpt from History of the metre.[edit]

An early definition of the metre was one ten-millionth of the Earth quadrant, the distance from the North Pole to the Equator, measured along a meridian through Paris.

The history of the metre starts with the Scientific Revolution that is considered to have begun with Nicolaus Copernicus's publication of De revolutionibus orbium coelestium in 1543. Increasingly accurate measurements were required, and scientists looked for measures that were universal and could be based on natural phenomena rather than royal decree or physical prototypes. Rather than the various complex systems of subdivision then in use, they also preferred a decimal system to ease their calculations.

With the French Revolution (1789) came a desire to replace many features of the Ancien Régime, including the traditional units of measure. As a base unit of length, many scientists had favoured the seconds 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. A new unit of length, the metre was introduced – defined as one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris, assuming an Earth flattening of ⁠1/334⁠.

The historical French official standard of the metre was made available in the form of the Mètre des Archives, a platinum bar held in Paris. During the mid nineteenth century, following the American Revolution and independence of Latin America, the metre gained adoption in Americas, particularly in scientific usage, and it was officially established as an international measurement unit by the Metre Convention of 1875 at the beginning of the Second Industrial Revolution.

The Mètre des Archives and its copies such as the Committee Meter were replaced from 1889 at the initiative of the International Geodetic Association by thirty platinum-iridium bars kept across the globe. A better standardization of the new prototypes of the metre and their comparison with each other and with the historical standard involved the development of specialized measuring equipment and the definition of a reproducible temperature scale.

Progress in science finally allowed the definition of the metre to be dematerialized; thus in 1960 a new definition based on a specific number of wavelengths of light from a specific transition in krypton-86 allowed the standard to be universally available by measurement. In 1983 this was updated to a length defined in terms of the speed of light; this definition was reworded in 2019:

The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit m⋅s⁻¹, where the second is defined in terms of the caesium frequency Δν_Cs.

Where older traditional length measures are still used, they are now defined in terms of the metre – for example the yard has since 1959 officially been defined as exactly 0.9144 metre.

Early adoptions of the metre internationally

In France, the metre was adopted as an exclusive measure in 1801 under the Consulate. This continued under the First French Empire until 1812, when Napoleon decreed the introduction of the non-decimal mesures usuelles, which remained in use in France up to 1840 in the reign of Louis Philippe. Meanwhile, the metre was adopted by the Republic of Geneva. After the joining of the canton of Geneva to Switzerland in 1815, Guillaume Henri Dufour published the first official Swiss map, for which the metre was adopted as the unit of length.

Adoption dates by country

• France: 1801–1812, then 1840
• Republic of Geneva, Switzerland: 1813
• Kingdom of the Netherlands: 1820
• Kingdom of Belgium: 1830
• Chile: 1848
• Kingdom of Sardinia, Italy: 1850
• Spain: 1852
• Portugal: 1852
• Colombia: 1853
• Ecuador: 1856
• Mexico: 1857
• Brazil: 1862
• Argentina: 1863
• Italy: 1863
• United States: 1866
• German Empire, Germany: 1872
• Austria, 1875
• Switzerland: 1877

SI prefixed forms of metre

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), or parsecs (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 terms micron and millimicron have been used instead of micrometre (μm) and nanometre (nm), respectively, but this practice is discouraged.

SI multiples of metre (m)

Submultiples			Multiples
Value	SI symbol	Name	Value	SI symbol	Name
10^−1 m	dm	decimetre	10^1 m	dam	decametre
10^−2 m	cm	centimetre	10^2 m	hm	hectometre
10^−3 m	mm	millimetre	10^3 m	km	kilometre
10^−6 m	μm	micrometre	10^6 m	Mm	megametre
10^−9 m	nm	nanometre	10^9 m	Gm	gigametre
10^−12 m	pm	picometre	10^12 m	Tm	terametre
10^−15 m	fm	femtometre	10^15 m	Pm	petametre
10^−18 m	am	attometre	10^18 m	Em	exametre
10^−21 m	zm	zeptometre	10^21 m	Zm	zettametre
10^−24 m	ym	yoctometre	10^24 m	Ym	yottametre
10^−27 m	rm	rontometre	10^27 m	Rm	ronnametre
10^−30 m	qm	quectometre	10^30 m	Qm	quettametre

Equivalents in other units

Metric unit expressed in non-SI units					Non-SI unit expressed in metric units
1 metre	≈	1.0936	yard		1 yard	=	0.9144	metre
1 metre	≈	39.370	inches		1 inch	=	0.0254	metre
1 centimetre	≈	0.39370	inch		1 inch	=	2.54	centimetres
1 millimetre	≈	0.039370	inch		1 inch	=	25.4	millimetres
1 metre	=	10^10	ångström		1 ångström	=	10^−10	metre
1 nanometre	=	10	ångström		1 ångström	=	100	picometres

Within this table, "inch" and "yard" mean "international inch" and "international yard" 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 ⁠5 000/127⁠ inches and to ⁠1 250/1 143⁠ yards.

A simple mnemonic to assist with conversion is "three 3s": 1 metre is nearly equivalent to 3 feet 3+3⁄8 inches. This gives an overestimate of 0.125 mm.

The ancient Egyptian cubit was about 0.5 m (surviving rods are 523–529 mm). Scottish and English definitions of the ell (2 cubits) were 941 mm (0.941 m) and 1143 mm (1.143 m) respectively. The ancient Parisian toise (fathom) was slightly shorter than 2 m and was standardised at exactly 2 m in the mesures usuelles system, such that 1 m was exactly 1⁄2 toise. The Russian verst was 1.0668 km. The Swedish mil was 10.688 km, but was changed to 10 km when Sweden converted to metric units.

See also

Wikimedia Commons has media related to Metre.

Look up metre in Wiktionary, the free dictionary.

• ISO 1 – standard reference temperature for length measurements
• Metric prefix
• Vertical position

Notes

1. "Base unit definitions: Meter". National Institute of Standards and Technology. Retrieved 28 September 2010.
2. International Bureau of Weights and Measures (20 May 2019), The International System of Units (SI) (PDF) (9th ed.), ISBN 978-92-822-2272-0, archived from the original on 18 October 2021
3. "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.
4. The most recent official brochure about the International System of Units (SI), written in French by the 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 (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 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). In 2008, NIST published the US version (Taylor and Thompson, 2008a) of the English text of the eighth edition of the BIPM publication 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 (Taylor and Thompson (2008a), p. iii). The Director of the NIST officially recognised this publication, together with Taylor and Thompson (2008b), as the "legal interpretation" of the SI for the United States (Turner). Thus, the spelling metre is referred to as the "international spelling"; the spelling meter, as the "American spelling".
5. Naughtin, Pat (2008). "Spelling metre or meter" (PDF). Metrication Matters. Archived from the original on 11 October 2016. Retrieved 12 March 2017.
6. "Meter vs. metre". Grammarist. 21 February 2011. Retrieved 12 March 2017.
7. The Philippines uses 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 (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).
8. Cambridge Advanced Learner's Dictionary. Cambridge University Press. 2008. Archived from the original on 3 July 2013. Retrieved 19 September 2012., s.v. ammeter, meter, parking meter, speedometer.
9. American Heritage Dictionary of the English Language (3rd ed.). Boston: Houghton Mifflin. 1992., s.v. meter.
10. "-meter – definition of -meter in English". Oxford Dictionaries. Archived from the original on 26 April 2017.
11. μετρέω. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project.
12. μέτρον in Liddell and Scott.
13. "History – The BIPM 150". Retrieved 24 January 2025.
14. Oxford English Dictionary, Clarendon Press 2nd ed. 1989, vol. IX p. 697 col. 3.
15. "BIPM – Commission internationale du mètre". www.bipm.org. Archived from the original on 18 November 2018. Retrieved 13 November 2019.
16. "BIPM – la définition du mètre". www.bipm.org. Archived from the original on 30 April 2017. Retrieved 17 June 2019.
17. 9th edition of the SI Brochure, BIPM, 2019, p. 131
18. Nelson, Robert A. (December 1981). "Foundations of the international system of units (SI)" (PDF). The Physics Teacher. 19 (9): 596–613. Bibcode:1981PhTea..19..596N. doi:10.1119/1.2340901.
19. Larousse, Pierre (1866–1877). Grand dictionnaire universel du XIXe siècle : français, historique, géographique, mythologique, bibliographique.... T. 11 MEMO-O / par M. Pierre Larousse. p. 163.
20. "Metrisches System". hls-dhs-dss.ch (in German). Retrieved 15 December 2021.
21. "Kartografie". hls-dhs-dss.ch (in German). Retrieved 13 December 2021.
22. Dufour, G.-H. (1861). "Notice sur la carte de la Suisse dressée par l'État Major Fédéral". Le Globe. Revue genevoise de géographie. 2 (1): 5–22. doi:10.3406/globe.1861.7582.
23. "Metrisches System". hls-dhs-dss.ch (in German). Retrieved 9 December 2021.
24. "Metric Act of 1866 – US Metric Association". usma.org. Retrieved 15 March 2021.
25. Taylor & Thompson 2003, p. 11.
26. Astin & Karo 1959.
27. Arnold Dieter (1991). Building in Egypt: pharaonic stone masonry. Oxford: Oxford University Press. ISBN 978-0-19-506350-9. p.251.
28. "Dictionary of the Scots Language". Archived from the original on 21 March 2012. Retrieved 6 August 2011.
29. The Penny Magazine of the Society for the Diffusion of Useful Knowledge. Charles Knight. 6 June 1840. pp. 221–22.
30. Hallock, William; Wade, Herbert T (1906). "Outlines of the evolution of weights and measures and the metric system". London: The Macmillan Company. pp. 66–69.
31. Cardarelli 2004.
32. Hofstad, Knut. "Mil". Store norske leksikon. Retrieved 18 October 2019.

References

• Alder, Ken (2002). The Measure of All Things : The Seven-Year Odyssey and Hidden Error That Transformed the World. New York: Free Press. ISBN 978-0-7432-1675-3.
• 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)
• Judson, Lewis V. (1 October 1976) [1963]. Barbrow, Louis E. (ed.). Weights and Measures Standards of the United States, a brief history. Derived from a prior work by Louis A. Fisher (1905). US: US Department of Commerce, National Bureau of Standards. doi:10.6028/NBS.SP.447. LCCN 76-600055. NBS Special Publication 447; NIST SP 447; 003-003-01654-3.
• Bigourdan, Guillaume (1901). 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 [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]. Paris: Gauthier-Villars.
• Clarke, Alexander Ross; Helmert, Friedrich Robert (1911b). "Earth, Figure of the" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 8 (11th ed.). Cambridge University Press. pp. 801–813.
• Guedj, Denis (2001). La Mesure du Monde [The Measure of the World]. Translated by Goldhammer, Art. Chicago: University of Chicago Press.
• Cardarelli, François (2003). "Chapter 2: The International system of Units" (PDF). Encydopaedia of scientific units, weights, and measures: their SI equivalences and origins. Springer-Verlag London Limited. Table 2.1, p. 5. ISBN 978-1-85233-682-0. Retrieved 26 January 2017. Data from Giacomo, P., Du platine à la lumière [From platinum to light], Bull. Bur. Nat. Metrologie, 102 (1995) 5–14.
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• Historical context of the SI: Meter. Retrieved 26 May 2010.
• National Institute of Standards and Technology. (27 June 2011). NIST-F1 Cesium Fountain Atomic Clock. Author.
• National Physical Laboratory. (25 March 2010). Iodine-Stabilised Lasers. Author.
• "Maintaining the SI unit of length". National Research Council Canada. 5 February 2010. Archived from the original on 4 December 2011.
• Republic of the Philippines. (2 December 1978). Batas Pambansa Blg. 8: An Act Defining the Metric System and its Units, Providing for its Implementation and for Other Purposes. Author.
• Republic of the Philippines. (10 October 1991). Republic Act No. 7160: The Local Government Code of the Philippines. Author.
• 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 publication Le Système International d' Unités (SI) (Special Publication 330). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 18 August 2008.
• Taylor, B.N. and Thompson, A. (2008b). 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.
• Turner, J. (deputy director of the National Institute of Standards and Technology). (16 May 2008). "Interpretation of the International System of Units (the Metric System of Measurement) for the United States". Federal Register Vol. 73, No. 96, p. 28432–28433.
• Zagar, B.G. (1999). Laser interferometer displacement sensors in J.G. Webster (ed.). The Measurement, Instrumentation, and Sensors Handbook. CRC Press. ISBN 0-8493-8347-1.