Metre

metreGeneral informationUnit systemSIUnit oflengthSymbolm[1]Conversions 1 m[1] in ...... is equal to ... Imperial/US units Nautical units ≈ 0.00053996 nmi

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.[2]

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 ΔνCs. 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.

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 use 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) ((I) measure, count or compare)[11] and noun μέτρον (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 motto ΜΕΤΡΩ ΧΡΩ (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.[13] In English, the use of the word metre (for the French unit mètre) began at least as early as 1797.[14]

In May 2025, the 150th anniversary of the Metre Convention was celebrated with events in Paris and Versailles, organised 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), 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.[27]

SI multiples of metre (m) Submultiples Multiples Value SI symbol Name Value SI symbol Name 10−1 m dm decimetre 101 m dam decametre 10−2 m cm centimetre 102 m hm hectometre 10−3 m mm millimetre 103 m km kilometre 10−6 m μm micrometre 106 m Mm megametre 10−9 m nm nanometre 109 m Gm gigametre 10−12 m pm picometre 1012 m Tm terametre 10−15 m fm femtometre 1015 m Pm petametre 10−18 m am attometre 1018 m Em exametre 10−21 m zm zeptometre 1021 m Zm zettametre 10−24 m ym yoctometre 1024 m Ym yottametre 10−27 m rm rontometre 1027 m Rm ronnametre 10−30 m qm quectometre 1030 m Qm quettametre Metric unitexpressed in non-SI units Non-SI unitexpressed 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 = 1010 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"[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 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).[29] Scottish and English definitions of the ell (2 cubits) were 941 mm (0.941 m) and 1143 mm (1.143 m) respectively.[30][31] 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.[32] The Russian verst was 1.0668 km.[33] The Swedish mil was 10.688 km, but was changed to 10 km when Sweden converted to metric units.[34]

• ISO 1 - standard reference temperature for length measurements
• Metric prefix
• Metrication