The metre (British spelling and BIPM spelling^{[1]}) or meter (American spelling) (from the French unit mètre, from the Greek noun μέτρον, "measure") is the base unit of length in some metric systems, including the International System of Units (SI). The SI unit symbol is m.^{[2]} The metre is defined as the length of the path travelled by light in a vacuum in 1/299 792 458 second.^{[1]}
The metre was originally defined in 1793 as one tenmillionth of the distance from the equator to the North Pole. In 1799, it was redefined in terms of a prototype metre bar (the actual bar used was changed in 1889). In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton86. In 1983, the current definition was adopted.
The imperial inch is defined as 0.0254 metres (2.54 centimetres or 25.4 millimetres). One metre is about 3 ^{3}⁄_{8} inches longer than a yard, i.e. about 39 ^{3}⁄_{8} inches.
Metre is the standard spelling of the metric unit for length in nearly all Englishspeaking nations except the United States^{[3]}^{[4]}^{[5]} and the Philippines,^{[6]} which use meter. Other Germanic languages, such as German, Dutch, and the Scandinavian languages^{[7]} likewise spell the word meter.
Measuring devices (such as ammeter, 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 of metre can be traced to the Greek verb μετρέω (metreo) (to 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 motto ΜΕΤΡΩ ΧΡΩ (metro chro) in the seal of the International Bureau of Weights and Measures (BIPM), which was a saying of the Greek statesman and philosopher Pittacus of Mytilene and may be translated as "Use measure!", thus calls for both measurement and moderation.
In 1668 the English cleric and philosopher John Wilkins proposed in an essay a decimalbased unit of length, the universal measure or standard based on a pendulum with a twosecond period.^{[11]} The use of the seconds pendulum to define length had been suggested to the Royal Society in 1660 by Christopher Wren. Christiaan Huygens had observed that length to be 38 Rijnland inches or 39.26 English inches; that is, 997 mm.^{[11]}^{[12]}^{[13]} No official action was taken regarding these suggestions.
In 1670 Gabriel Mouton, Bishop of Lyon, also suggested a universal length standard with decimal multiples and divisions, to be based on a oneminute angle of the Earth's meridian arc or (as the Earth's circumference was not easy to measure) on a pendulum with a twosecond period. In 1675, the Italian scientist Tito Livio Burattini, in his work Misura Universale, used the phrase metro cattolico ("universal measure"), derived from the Greek μέτρον καθολικόν (métron katholikón), to denote the standard unit of length derived from a pendulum.^{[14]} As a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. On 7 October 1790 that commission advised the adoption of a decimal system, and on 19 March 1791 advised the adoption of the term mètre ("measure"), a basic unit of length, which they defined as equal to one tenmillionth of the distance between the North Pole and the Equator.^{[15]}^{[16]}^{[17]}^{[18]} In 1793, the French National Convention adopted the proposal; this use of metre in English began at least as early as 1797.^{[19]}
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In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because the force of gravity varies slightly over the surface of the Earth, which affects the period of a pendulum.
To establish a universally accepted foundation for the definition of the metre, more accurate measurements of this meridian were needed. The French Academy of Sciences commissioned an expedition led by Jean Baptiste Joseph Delambre and Pierre Méchain, lasting from 1792 to 1799, which attempted to accurately measure the distance between a belfry in Dunkerque and Montjuïc castle in Barcelona to estimate the length of the meridian arc through Dunkerque. This portion of the meridian, assumed to be the same length as the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator. The problem with this approach is that the exact shape of the Earth is not a simple mathematical shape, such as a sphere or oblate spheroid, at the level of precision required for defining a standard of length. The irregular and particular shape of the Earth smoothed to sea level is represented by a mathematical model called a geoid, which literally means "Earthshaped". Despite these issues, in 1793 France adopted this definition of the metre as its official unit of length based on provisional results from this expedition. However, it was later determined that the first prototype metre bar was short by about 200 micrometres because of miscalculation of the flattening of the Earth, making the prototype about 0.02% shorter than the original proposed definition of the metre. Regardless, this length became the French standard and was progressively adopted by other countries in Europe.
The expedition was fictionalised in Denis Guedj, Le mètre du Monde.^{[20]} Ken Alder wrote factually about the expedition in The Measure of All Things: the seven year odyssey and hidden error that transformed the world.^{[21]}
In 1867 at the second general conference of the International Association of Geodesy held in Berlin, the question of an international standard unit of length was discussed in order to combine the measurements made in different countries to determine the size and shape of the Earth.^{[22]}^{[23]}^{[24]} The conference recommended the adoption of the metre and the creation of an international metre commission, according to the proposal of Johann Jacob Baeyer, Adolphe Hirsch and Carlos Ibáñez e Ibáñez de Ibero.^{[22]}
In the 1870s and in light of modern precision, a series of international conferences was held to devise new metric standards. The Metre Convention (Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres, France. This new organisation was to construct and preserve a prototype metre bar, distribute national metric prototypes, and maintain comparisons between them and nonmetric measurement standards. The organisation created such a bar in 1889 at the first General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), establishing the International Prototype Metre as the distance between two lines on a standard bar composed of an alloy of 90% platinum and 10% iridium, measured at the melting point of ice.^{[25]}
The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889.
In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of length. By 1925, interferometry was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new International System of Units (SI) as equal to 1 650 763.73 wavelengths of the orangered emission line in the electromagnetic spectrum of the krypton86 atom in a vacuum.^{[26]}
To further reduce uncertainty, the 17th CGPM in 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of the second and the speed of light:^{[1]}
This definition fixed the speed of light in vacuum at exactly 792458 metres per second (≈ 299000 km/s). An intended byproduct of the 17th CGPM's definition was that it enabled scientists to compare lasers accurately using frequency, resulting in wavelengths with onefifth the uncertainty involved in the direct comparison of wavelengths, because interferometer errors were eliminated. To further facilitate reproducibility from lab to lab, the 17th CGPM also made the iodinestabilised 300helium–neon laser "a recommended radiation" for realising the metre.^{[27]} For the purpose of delineating the metre, the BIPM currently considers the HeNe laser wavelength, λ_{HeNe}, to be 21258 nm with an estimated relative standard uncertainty (U) of 632.991×10^{−11}.^{[27]}^{[28]}^{[29]} This uncertainty is currently one limiting factor in laboratory realisations of the metre, and it is several orders of magnitude poorer than that of the second, based upon the caesium fountain 2.1atomic clock (U = ×10^{−16} 5).^{[30]} Consequently, a realisation of the metre is usually delineated (not defined) today in labs as 579800.762042(33) wavelengths of heliumneon laser light in a vacuum, the error stated being only that of frequency determination.^{[27]} This bracket notation expressing the error is explained in the article on 1measurement uncertainty.
Practical realisation of the metre is subject to uncertainties in characterising the medium, to various uncertainties of interferometry, and to uncertainties in measuring the frequency of the source.^{[31]} A commonly used medium is air, and the National Institute of Standards and Technology (NIST) has set up an online calculator to convert wavelengths in vacuum to wavelengths in air.^{[32]} As described by NIST, in air, the uncertainties in characterising the medium are dominated by errors in measuring temperature and pressure. Errors in the theoretical formulas used are secondary.^{[33]} By implementing a refractive index correction such as this, an approximate realisation of the metre can be implemented in air, for example, using the formulation of the metre as 579800.762042(33) wavelengths of helium–neon laser light in vacuum, and converting the wavelengths in a vacuum to wavelengths in air. Air is only one possible medium to use in a realisation of the metre, and any 1partial vacuum can be used, or some inert atmosphere like helium gas, provided the appropriate corrections for refractive index are implemented.^{[34]}
The metre is defined as the path length travelled by light in a given time and practical laboratory length measurements in metres are determined by counting the number of wavelengths of laser light of one of the standard types that fit into the length,^{[37]} and converting the selected unit of wavelength to metres. Three major factors limit the accuracy attainable with laser interferometers for a length measurement:^{[31]}^{[38]}
Of these, the last is peculiar to the interferometer itself. The conversion of a length in wavelengths to a length in metres is based upon the relation
which converts the unit of wavelength λ to metres using c, the speed of light in vacuum in m/s. Here n is the refractive index of the medium in which the measurement is made, and f is the measured frequency of the source. Although conversion from wavelengths to metres introduces an additional error in the overall length due to measurement error in determining the refractive index and the frequency, the measurement of frequency is one of the most accurate measurements available.^{[38]}
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Basis of definition  Date  Absolute uncertainty 
Relative uncertainty 

1/10 000 000 part of the quadrant along the meridian, measurement by Delambre and Méchain (443.296 lines)  1795  500–100 μm  10^{−4} 
First prototype Mètre des Archives platinum bar standard  1799  50–10 μm  10^{−5} 
Platinumiridium bar at melting point of ice (1st CGPM)  1889  0.2–0.1 μm (200–100 nm)  10^{−7} 
Platinumiridium bar at melting point of ice, atmospheric pressure, supported by two rollers (7th CGPM)  1927  n.a.  n.a. 
Hyperfine atomic transition; 650763.73 wavelengths of light from a specified transition in 1krypton86 (11th CGPM)  1960  4 nm  ×10^{−9}^{[43]} 4 
Length of the path travelled by light in a vacuum in 1/299 792 458 second (17th CGPM)  1983  0.1 nm  10^{−10} 
SI prefixes are often employed to denote decimal multiples and submultiples of the metre, as shown in the table below. As indicated in the table, some are commonly used, while others are not. Long distances are usually expressed in km, astronomical units (149.6 Gm), lightyears (10 Pm), or parsecs (31 Pm), rather than in Mm, Gm, Tm, Pm, Em, Zm or Ym; "30 cm", "30 m", and "300 m" are more common than "3 dm", "3 dam", and "3 hm", respectively.
The terms micron and (occasionally) millimicron are often used instead of micrometre (μm) and nanometre (nm), but this practice is officially discouraged.^{[44]}
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  
Common prefixed units are in bold face. 
Metric unit expressed in nonSI units 
NonSI 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  ≈  70 0.393  inch  1 inch  ≡  2.54  centimetres  
1 millimetre  ≈  370 0.039  inch  1 inch  ≡  25.4  millimetres  
1 metre  ≡  1 × 10^{10}  ångström  1 ångström  ≡  1 × 10^{−10}  metre  
1 nanometre  ≡  10  ångström  1 ångström  ≡  100  picometres 
Within this table, "inch" and "yard" mean "international inch" and "international yard"^{[45]} respectively, though approximate conversions in the left column hold for both international and survey units.
One metre is exactly equivalent to 10 000/254 inches and to 10 000/9 144 yards.
A simple mnemonic aid exists to assist with conversion, as three "3"s:
The ancient Egyptian cubit was about 0.5 m (surviving rods are 523–529 mm). Scottish and English definitions of the ell (two 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.
An identical metric system to that introduced in France was proposed in 1668 by Bishop John Wilkins, a founder of the Royal Society in England. ... he proposed an integrated system of measurement based on a decimal system and almost identical to the modern metric system. His unit of measurement was 997 millimeters  almost exactly a meter.
he [Wilkins] proposed essentially what became ... the French decimal metric system
The error [introduced by using air] can be reduced tenfold if the chamber is filled with an atmosphere of helium rather than air.
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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