This article is about the unit of length. For other uses of metre or meter, see meter (disambiguation).

The metre, symbol: m, is the basic unit of distance (or of "length", in the parlance of the physical sciences) in the International System of Units. The internationally-accepted spelling of the unit in English is "metre", although the American English spelling meter is a common variant. However, both American and non-American forms of English agree that the spelling "meter" should be used as a suffix in the names of measuring devices such as chronometers and micrometers.

A metre is defined as the length of the path travelled by light in an absolute vacuum during a time interval of exactly 1/299,792,458 of a second. This definition does not change the size of the unit (see History below), but it was introduced to take into account recent developments in measurement techniques whereby length and time can be reproduced with very high accuracy — in the case of time, to an accuracy of 1013. One metre is equal to approximately 39.37 inches (3.28 feet).



The means of defining the metre has changed over time:

  • 1793: 1/10,000,000 of the distance from the pole to the equator.
  • 1795: Provisional metre bar constructed in brass.
  • 1799: Definitive prototype metre bars constructed in platinum.
  • 1889: International prototype metre bar in platinum-iridium, cross-section X.
  • 1960: Krypton spectrum: 1650763.73 wavelengths in vacuum of the radiation corresponding to the transition between levels 2p10 and 5d5 of the krypton-86 atom.
  • 1983: Speed of light definiton: Length travelled by light in vacuum during 1/299792458 of a second.

The word itself is from the Greek metron (μετρον), "a measure" via the French mtre. Its first recorded usage in English is from 1797.

In the eighteenth century, there were two favoured approaches to the definition of the standard unit of length. One suggested defining the metre as the length of a pendulum with a half-period of one second. The other suggested defining the metre as one ten-millionth of the length of the earth's meridian along a quadrant (one-fourth the polar circumference of the earth). In 1791, the French Academy of Sciences selected the meridional definition, using the meridian of Paris, over the pendular definition because of the slight variation of the force of gravity over the surface of the earth, which affects the period of a pendulum.

Missing image
International Prototype Metre standard bar made of platinum-iridium. This was the standard until 1960, when the new SI system used a krypton-spectrum measurement as the base. In 1983 the current metre was defined by a relationship to the speed of light in a vacuum. (NIST)

In August 1793, the Republican Government in France decreed that the standard unit of length would be <math>10^{-7}<math> of the earth's quadrant passing through Paris and that the unit be called the metre. Five years later the survey of the arc was completed and three platinum standards and several iron copies were made. Subsequent analysis showed that the length of the earth's quadrant had been incorrectly surveyed resulting in the first prototype metre bar being short by a fifth of a millimetre (due to miscalculation of the flattening of the earth), instead of altering the length of the metre to maintain the <math>10^{-7}<math> ratio, the metre was redefined as the distance between two marks on a bar. So, the circumference of the Earth through the poles is only approximately forty million metres.

In the 1870s and in light of modern precision, a series of international conferences were held to devise new metric standards. The Convention du Mtre (Treaty of the Metre) 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 Svres, France. This new organisation would preserve the new prototype metre and kilogram when constructed, and would maintain comparisons between them and the national standards for the metre and the kilogram. This organisation created a new prototype bar in 1889, establishing the International Prototype Metre as the distance between two lines on its standard-issue bar of an alloy of ninety percent platinum and ten percent iridium. 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 distance. By 1925, interferometry was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960. The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889.

The eleventh General Conference on Weights and Measures (CGPM: Confrence gnrale des poids et mesures) in 1960 defined the metre in the new SI system as equal to 1,650,763.73 wavelengths of the orange-red emission line in the spectrum of the krypton-86 atom in a vacuum.

To further reduce uncertainty, the seventeenth CGPM of 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of time and the speed of light:

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Note that this definition exactly fixes the speed of light in a vacuum at 299,792,458 metres per second. Definitions based on the physical properties of light are more precise and reproducible because the properties of light are considered to be universally constant.

Orders of magnitude

  • A decimetre (American spelling: decimeter, symbol: dm) is a measurement of distance equal to ten centimetres or 1/10 metre. Although considered obsolescent, Canada uses it as the metric measurement for the loading of ships (see Plimsoll line) because measurement in centimetres is too precise.
  • A centimetre (American spelling: centimeter, symbol: cm) is one-hundredth of one metre
  • A millimetre (American spelling: millimeter, symbol mm) is equal to one thousandth of a metre.
    • 1 mm is equal to about 0.03937 inches
    • The level of rainfall is also reported as millimeters as measured with a rain gauge.
  • A micrometre (American spelling: micrometer, symbol m) is defined as one millionth of a metre (1×10−6 m)
    • The symbol "um" is sometimes used, when the and μ are not available, e.g. when using a typewriter.
    • The micrometre is a common unit of measurement for wavelengths of infrared radiation.
    • Some people (especially in astronomy and the semiconductor business) use the old name micron and/or the solitary symbol for the same thing, even though it is officially discouraged. They were official in 1879–1967.
    • A micrometer is also a name for a measuring instrument.
  • A nanometre (American spelling: nanometer, symbol: nm) is 1.0×10−9 metres
    • commonly used in measuring the wavelengths of visible light (400 nm to 700 nm), ultraviolet radiation and gamma rays; amongst other things.
    • In older (e.g. 1958) texts, the nm is sometimes denoted "mµ" for "millimicron", based on the old term "micron" for a micrometre. Double prefixes are not allowed in SI, and this is never seen in modern writing or speech.
  • Picometre (American spelling: picometer, symbol: pm) is equal to 10−12 of a metre.
    • It is commonly used in measuring atomic-scale distances; atom diameters are in the range from ~30 to 600 pm.
  • Femtometre (American spelling: femtometer, symbol:fm) is equal to 10−15 (femto) of a metre.
    • It is commonly used in measuring the diameter of atomic nuclei. The diameter of an atomic nucleus is up to about 15 fm. Neutrons and protons are about 1 fm in diameter.
    • In the parlance of particle physicists, a femtometre is often called a fermi (same symbol), after the physicist Enrico Fermi.
  • Attometre (American spelling: attometer, symbol: am) is equal to 10−18, or 1 quintillionth (atto-) of a metre.
    • An electron's radius is thought to be about 1 am.
  • Zeptometre (American spelling: zeptometer, symbol: zm) is equal to 10-21 (zepto-) of a metre.
  • Yoctometre (American spelling: yoctometer, symbol: ym) is equal to 10-24 (yocto) of a metre.


Unicode has symbols for "cm" (㎝), for square centimetre (㎠) and for cubic centimetre (㎤); also, for "mm" (㎜), for square millimetre (㎟) and for cubic millimetre (㎣); however, they are useful only with East Asian fixed-width CJK fonts, because they are equal in size to one Chinese character.


  • A Dictionary of Scientific Units - including dimensionless numbers and scales. 5th Edition 1986. H.G. Jerrard and D.B. McNeill.

See also

External links

bg:Метър ca:Metre cs:Metr da:Meter de:Meter et:Meeter es:Metro eo:Metro fr:Mtre gl:Metro ko:미터 hr:Metar id:Meter ia:Metro is:Metri it:Metro la:Metrum he:מטר hu:Mter ms:Meter nl:Meter ja:メートル no:Meter nn:Meter pl:Metr pt:Metro ro:Metru ru:Метр simple:Metre sk:Meter sl:Meter sr:Метар fi:Metri sv:Meter ta:மீட்டர் th:เมตร vi:Mt uk:Метр zh:米 (计量单位)


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