Unit of length

Short description: Reference value of length

A ruler, depicting two customary units of length, the centimeter and the inch

A unit of length is any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary units are also in use. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.[1][2][3]

## Contents

* 1 History
* 2 Metric system
* 2.1 SI
* 2.2 Non-SI
* 3 Imperial/U.S.
* 4 Marine
* 5 Aviation
* 6 Surveying
* 6.1 Australian building trades
* 6.2 Surveyor's trade
* 7 Science
* 7.1 Astronomy
* 7.2 Physics
* 8 Archaic
* 9 Informal
* 10 Other
* 11 See also
* 12 References
* 13 Further reading

## History

This section is an excerpt from History of measurement#Units of length[ edit ]

## Metric system

### SI

See also: Orders of magnitude (length)

The base unit in the International System of Units (SI) is the metre, defined as "the length of the path travelled by light in vacuum during a time interval of ​1⁄299792458 seconds."[4] It is approximately equal to 1.0936 yd. Other SI units are derived from the metre by adding prefixes, as in millimetre or kilometre, thus producing systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. For example, a kilometre is 1000 m.

### Non-SI

In the centimeter–gram–second system of units, the basic unit of length is the centimeter, or ​1⁄100 of a meter. Other non-SI units are derived from decimal multiples of the meter.

Name  | Symbol  | SI value

|
|

fermi  | fm  | 1 femtometer
ångström | Å  | 100 picometers
micron  | μm  | 1 micrometer
Norwegian/Swedish mil or myriameter  |  | 10,000 meters
x unit | xu  | 0.1 picometer

## Imperial/U.S.

The basic unit of length in the imperial and U.S. customary systems is the yard, defined as exactly 0.9144 m by international treaty in 1959.[2][5]

Common imperial units and U.S. customary units of length include:[6]

* thou or mil (​1⁄1000 of an inch)
* inch (25.4 mm)
* foot (12 inches, 0.3048 m)
* yard (3 feet, 0.9144 m)
* (terrestrial) mile (5280 feet, or 1760 yards 1609.344 m)
* (land) league 3 miles (4,800 m)

## Marine

In addition, the following are used by sailors:

* fathom (for depth; only in non-metric countries) (2 yards = 1.8288 m)
* nautical mile (one minute of arc of latitude = 1852 m)

## Aviation

Air traffic control uses feet for altitude worldwide,[7]: Chapter 4 [8]: Altitude, Flight Level except in a minority of countries including Russia, China, North Korea, and many CIS countries.[9] Distance is measured in nautical miles.[7]: Chapter 4 [8]: Distance (DME)

## Surveying

Determination of the rod, using the length of the left foot of 16 randomly chosen people coming from church service

Surveyors in the United States continue to use:

* chain (22 yards, or 20.1168 m)
* rod (also called pole or perch) (quarter of a chain, 5​1⁄2 yards, or 5.0292 m)

### Australian building trades

The Australian building trades adopted the metric system in 1966 and the units used for measurement of length are meters (m) and millimeters (mm). Centimeters (cm) are avoided as they cause confusion when reading plans. For example, the length two and a half meters is usually recorded as 2500 mm or 2.5 m; it would be considered non-standard to record this length as 250 cm.[10][11]

### Surveyor's trade

American surveyors use a decimal-based system of measurement devised by Edmund Gunter in 1620. The base unit is Gunter's chain of 66 feet (20 m) which is subdivided into 4 rods, each of 16.5 ft or 100 links of 0.66 feet. A link is abbreviated "lk", and links "lks", in old deeds and land surveys done for the government.

## Science

### Astronomy

Astronomical measure uses:

* Earth radius R⊕ ≈ 6,371 km[12]
* Lunar distance LD ≈ 384402 km.[13] Average distance between the center of Earth and the center of the Moon.
* astronomical unit au. Defined as 149597870700 m.[14] Approximately the distance between the Earth and Sun.
* light-year ly ≈ 9460730472580.8 km. The distance that light travels in a vacuum in one Julian year.[15]
* parsec pc ≈ 30856775814671.9 km or about 3.26156 ly
* Hubble length 14.4 billion light-years or 4.55 gigaparsecs

### Physics

In atomic physics, sub-atomic physics, and cosmology, the preferred unit of length is often related to a chosen fundamental physical constant, or combination thereof. This is often a characteristic radius or wavelength of a particle. Some common natural units of length are included in this table:

Atomic property  | Symbol  | Length, in meters  | Reference

|
|
|

The classical electron radius  | re | 2.817940285(31)×10−15 | [16]
The Compton wavelength of the electron  | λC | 2.426310215(18)×10−12 | [16]
The reduced Compton wavelength of the electron  | ~~λ~~ C | 3.8615926764(18)×10−13 | [17]
The Compton wavelength (or reduced Compton wavelength) of any fundamental particle  | ~~λ~~ x |  |
The Bohr radius of the hydrogen atom (Atomic unit of length)  | a0 | 5.291772083(19)×10−11 | [16]
The reduced wavelength of hydrogen radiation  | 1 / R∞ | 9.112670505509(83)×10−8 | [16]
The Planck length | 𝓁P | 1.616199(97)×10−35 | [18]
Stoney unit of length  | lS | 1.381×10−35 |
Quantum chromodynamics (QCD) unit of length  | lQCD | 2.103×10−16 |
Natural units based on the electronvolt  | 1 eV−1 | 1.97×10−7 |

## Archaic

Archaic units of distance include:

* cana
* cubit
* rope
* league
* li (China)
* pace (the "double pace" of about 5 feet used in Ancient Rome)
* verst (Russia)

## Informal

See also: List of humorous units of measurement and List of unusual units of measurement

In everyday conversation, and in informal literature, it is common to see lengths measured in units of objects of which everyone knows the approximate width. Common examples are:

* Double-decker bus (9.5–11 meters in length)
* American football field (100 yards in length)
* Thickness of a human hair (around 80 micrometers)

## Other

Horse racing and other equestrian activities keep alive:

* furlong = 1⁄8 mile (201.168 m)
* horse length ≈ 8 feet (2.4 m)

## See also

* List of conversion factors § Length
* List of examples of lengths
* List of unusual units of measurement § Length
* Medieval weights and measures
* Orders of magnitude (length)
* System of measurement
* Units of measurement
* Units of Length list

## References

1. ↑ Cardarelli, François (2003). Encyclopaedia of Scientific Units, Weights, and Measures: Their SI Equivalences and Origins. Springer. ISBN 9781852336820. https://archive.org/details/encyclopaediaofs0000card.
2. ↑ 2.0 2.1 Hinkelman, Edward G.; Sibylla Putzi (2005). Dictionary Of International Trade: Handbook Of The Global Trade Community. World Trade Press. p. 245. ISBN 9781885073723.
3. ↑ Judson, Lewis Van Hagen (1960). Units of Weight and Measure (United States Customary and Metric): Definitions and Tables of Equivalents, Issue,233. U.S. Department of Commerce, National Bureau of Standards. pp. 3-4. https://books.google.com/books?id=Mialg1senzMC&q=Units+of+Weight+and+Measure+(United+States+Customary+and+Metric). Retrieved 16 October 2012.
4. ↑ "17th General Conference on Weights and Measures (1983), Resolution 1.". http://www.bipm.org/en/CGPM/db/17/1/.
5. ↑ Donald Fenna (2002). A dictionary of weights, measures, and units. Oxford University Press. pp. 130–131. ISBN 978-0198605225. https://archive.org/details/dictionaryofweig0000fenn. Retrieved 8 January 2012.
6. ↑ Cardarelli 2003, pp. 29–30
7. ↑ 7.0 7.1 International Civil Aviation Organization - International Standards and Recommended Practices - Annex 5 to the Convention on International Civil Aviation - Units of Measurement to be Used in Air and Ground Operations Fifth Edition - July 2010
8. ↑ 8.0 8.1 Federal Aviation Administration (29 March 2018). Pilot/Controller Glossary. pp. PCG A-10, PCG D-4, PCG F−3. https://www.faa.gov/air_traffic/publications/media/pcg_basic_chgs 1_2_9-13-18.pdf. Retrieved 20 March 2026.
9. ↑ "About the Transition to the ICAO Vertical Separation System and Reduced Vertical Separation Minimum (RVSM) From FL 290 TO FL 410 Inclusive in the Airspace of the Russian Federation Effective From 17 November 2011". rusaero.aero. 20 November 2011. http://www.rusaero.aero/files/679/aic-2011-11ang.pdf.
10. ↑ Wilks, Kevin Joseph. (1992). Metrication in Australia : a review of the effectiveness of policies and procedures in Australia's conversion to the metric system. Australia. Department of Industry, Technology, and Commerce.. Canberra: Australian Govt. Pub. Service. pp. 94. ISBN 0-644-24860-2. OCLC 27702954.
11. ↑ "Metrication in Australia". http://themetricmaven.com/wp-content/uploads/2012/03/Metrication-in-Australia-built-2013-06-24.pdf.
12. ↑ Moritz, H. (March 2000). "Geodetic Reference System 1980". Journal of Geodesy 74 (1): 128–133. doi:10.1007/s001900050278. Bibcode: 2000JGeod..74..128..
13. ↑ Battat, J. B. R.; Murphy, T. W.; Adelberger, E. G. (January 2009). "The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO): Two Years of Millimeter-Precision Measurements of the Earth-Moon Range". Astronomical Society of the Pacific 121 (875): 29–40. doi:10.1086/596748. Bibcode: 2009PASP..121...29B.
14. ↑ Geoff Brumfiel (14 Sep 2012). "The astronomical unit gets fixed: Earth–Sun distance changes from slippery equation to single number.". Nature. doi:10.1038/nature.2012.11416. http://www.nature.com/news/the-astronomical-unit-gets-fixed-1.11416. Retrieved 14 Sep 2012.
15. ↑ The IAU and astronomical units, International Astronomical Union, http://www.iau.org/public_press/themes/measuring/, retrieved 2008-07-05
16. ↑ 16.0 16.1 16.2 16.3 Quinn, T.J.; Leschiutta, S.; Tavella, P. (August 2000). Recent advances in metrology and fundamental constants. Amsterdam; Washington, DC: IOS Press, 2001. Proceedings of the International School of Physics "Enrico Fermi". pp. 142–143. ISBN 9781586031671. https://books.google.com/books?id=WE22Fez60EcC&q=bohr+ruler&pg=PA142.
17. ↑ "Compton wavelength over 2 pi". The NIST Reference on Constants, Units, and Uncertainty. NIST. http://physics.nist.gov/cgi-bin/cuu/Value?ecomwlbar.
18. ↑ "Planck length". The NIST Reference on Constants, Units, and Uncertainty. NIST. http://physics.nist.gov/cgi-bin/cuu/Value?plkl.

## Further reading

* Whitelaw, Ian (2007). A Measure of All Things: The Story of Man and Measurement. Macmillan. ISBN 9780312370268.

* v
* t
* e

Units of length used in Astronomy

* Astronomical system of units

* Earth radius (R⊕ or RE)
* Light-second (ls)
* Solar radius (R☉)
* gigametre (Gm)
* Astronomical unit (au)
* terametre (Tm)
* light-year (ly)
* parsec (pc)
* kiloparsec (kpc)
* megaparsec (Mpc)
* gigaparsec (Gpc)

See also
Cosmic distance ladder
Orders of magnitude (length)
Conversion of units

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