Kilometre

"km" redirects here. For other uses, see KM (disambiguation).

The kilometre (SI symbol: km; /ˈkɪləmiːtər/ or /kɪˈlɒmətər/), spelt kilometer in American and Philippine English, is a unit of length in the International System of Units (SI), equal to one thousand metres (kilo- being the SI prefix for 1000). It is the preferred measurement unit to express distances between geographical places on land in most of the world; notable exceptions are the United States and the United Kingdom where the statute mile is used.

kilometre
The main span of the 25 de Abril Bridge is 1.013 kilometres (1,013 m).
General information
Unit system	SI
Unit of	length
Symbol	km
Conversions
1 km in ...	... is equal to ...
SI base units	1000 m
imperial/US units	0.62137 mi  1093.6 yd  3280.8 ft
nautical units	0.53996 nmi

Pronunciation

There are two common pronunciations for the word.^[1]

1. /ˈkɪləmiːtər, -loʊ-/
2. /kɪˈlɒmɪtər/

The first pronunciation follows a pattern in English whereby SI units are pronounced with the stress on the first syllable (as in kilogram, kilojoule and kilohertz) and the pronunciation of the actual base unit does not change irrespective of the prefix (as in centimetre, millimetre, nanometre and so on). It is generally preferred by the British Broadcasting Corporation (BBC), the Canadian Broadcasting Corporation (CBC), and the Australian Broadcasting Corporation (ABC).^{[citation needed]}

Many other users, particularly in countries where SI (the metric system) is not widely used, use the second pronunciation with stress on the second syllable.^[2][3] The second pronunciation follows the stress pattern used for the names of measuring instruments (such as micrometer, barometer, thermometer, tachometer, and speedometer). The contrast is even more obvious in countries that use the American spelling of the word metre. This pronunciation is irregular because it makes the kilometre the only SI unit with the stress on the second syllable.

After Australia introduced the metric system in 1970, the first pronunciation was declared official by the government's Metric Conversion Board. However, the Australian prime minister at the time, Gough Whitlam, insisted that the second pronunciation was the correct one because of the Greek origins of the two parts of the word.^[4]

Equivalence to other units of length

1 kilometre	≡	1000	metres
	≈	3281	feet
	≈	1094	yards
	≈	0.621	miles
	≈	0.540	nautical miles
	≈	6.68×10−9	astronomical units[a]
	≈	1.06×10−13	light-years[b]
	≈	3.24×10−14	parsecs

History

Historical divisions of the meridian in France

By a decree of 8 May 1790, the French National Constituent Assembly ordered the French Academy of Sciences to develop a new measurement system. In August 1793, the French National Convention decreed the metre as the sole length measurement system in the French Republic and it was based on ⁠1/10⁠ millionth of the distance from the orbital poles (either North or South) to the Equator, this being a truly internationally based unit. The first name of the kilometre was "Millaire". Although the metre was formally defined in 1799, the myriametre (10000 metres) was preferred to the "kilometre" for everyday use. The term "myriamètre" appeared a number of times in the text of Develey's book Physique d'Emile: ou, Principes de la science de la nature,^[5] (published in 1802), while the term kilometre only appeared in an appendix. French maps published in 1835 had scales showing myriametres and "lieues de Poste" (Postal leagues of about 4288 metres).^[6]

The Dutch, on the other hand, adopted the kilometre in 1817 but gave it the local name of the mijl.^[7] It was only in 1867 that the term "kilometer" became the only official unit of measure in the Netherlands to represent 1000 metres.^[8]

Two German textbooks dated 1842^[9][10] and 1848^[11] respectively give a snapshot of the use of the kilometre across Europe: the kilometre was in use in the Netherlands and in Italy, and the myriametre was in use in France.

In 1935, the International Committee for Weights and Measures (CIPM) officially abolished the prefix "myria-" and with it the "myriametre", leaving the kilometre as the recognised unit of length for measurements of that magnitude.^[12]

The symbol km for the kilometre is in lower case and has been standardised by the BIPM.^[13][14] A slang term for the kilometre in the US, UK, and Canadian militaries is klick.^[15]

Kilometre records

Some sporting disciplines feature 1000 m (one-kilometre) races in major events (such as the Olympic Games). In some disciplines—although world records are catalogued—one-kilometre events remain a minority.

1 km world records for various sporting disciplines

Discipline	Name	Time (min:sec)	Location	Date	Comments
Running (M)	Noah Ngeny	2:11.96[16]	Rieti, Italy	5 Sep 1999	Not an Olympic event
Running (F)	Svetlana Masterkova	2:28.98[17]	Brussels	23 Aug 1996	Not an Olympic event
Speed skating (M)	Pavel Kulizhnikov	1:05.69	Salt Lake City	15 Feb 2020
Speed skating (F)	Brittany Bowe	1:11.61	Salt Lake City	9 Mar 2019
Track cycling (M)	François Pervis	56.303[18]	Aguascalientes, Mexico	7 Dec 2013	No official 1000 m women's record

See also

• Conversion of units, for comparison with other units of length
• Cubic metre
• Metric prefix
• Mileage
• Odometer
• Orders of magnitude (length)
• Square kilometre

Notes

1. One astronomical unit is currently accepted to be equal to 149597870691±30 m.
2. A light-year is equal to 9.4607304725808×10¹² km the distance light travels through vacuum in one year (365.25 days).