Unit of length

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

A unit of length refers to 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.<ref>Template:Cite book</ref><ref name=Hinkelman>Template:Cite book</ref><ref>Template:Cite book</ref>

Metric system

SI

Template:Main article Template:See also The base unit in the International System of Units (SI) is the meter, defined as "the length of the path travelled by light in vacuum during a time interval of Template:Frac seconds."<ref name="Res1">Template:Cite web</ref> It is approximately equal to Template:Val. Other SI units are derived from the meter by adding prefixes, as in millimeter or kilometer, thus producing systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. For example, a kilometer is Template:Val.

Non-SI

In the centimeter–gram–second system of units, the basic unit of length is the centimeter, or Template:Frac 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 Template:Val by international treaty in 1959.<ref name=Hinkelman/><ref name="Fenna2002">Template:Cite book</ref>

Common imperial units and U.S. customary units of length include:<ref>Template:Harvnb</ref>

• thou or mil (Template:Frac of an inch)
• inch (Template:Val)
• foot (12 inches, 0.3048 m)
• yard (3 feet, 0.9144 m)
• (terrestrial) mile (5280 feet, or 1760 yards 1609.344 m)
• (land) league Template:Convert

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 = Template:Val)

Aviation

Aviators use feet for altitude worldwide (except in Russia and China) and nautical miles for distance.Template:Citation needed

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 Template:Val)
• rod (also called pole or perch) (quarter of a chain, 5Template:Frac yards, or Template:Val)

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.<ref>Template:Cite book</ref><ref>Template:Cite web</ref>

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 Template:Convert 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 Template:Earth radius ≈ 6,371 km<ref name="Moritz2000">Template:Cite journal</ref>
• Lunar distance LD ≈ Template:Val.<ref name="Apache Point">Template:Cite journal</ref> Average distance between the center of Earth and the center of the Moon.
• astronomical unit au. Defined as Template:Val.<ref name=Nature2012>Template:Cite journal</ref> Approximately the distance between the Earth and Sun.
• light-year ly ≈ Template:Val. The distance that light travels in a vacuum in one Julian year.<ref name="IAUgen">Template:Citation</ref>
• parsec pc ≈ Template:Val or about Template:Val
• 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	Template:Val	<ref name="Bohr ruler">Template:Cite book</ref>
The Compton wavelength of the electron	λC	Template:Val	<ref name="Bohr ruler"/>
The reduced Compton wavelength of the electron	Template:StrikethroughC	Template:Val	<ref>Template:Cite web</ref>
The Compton wavelength (or reduced Compton wavelength) of any fundamental particle	Template:Strikethroughx
The Bohr radius of the hydrogen atom (Atomic unit of length)	a0	Template:Val	<ref name="Bohr ruler"/>
The reduced wavelength of hydrogen radiation	1 / R∞	Template:Val	<ref name="Bohr ruler"/>
The Planck length	𝓁P	Template:Val	<ref>Template:Cite web</ref>
Stoney unit of length	lS	Template:Val
Quantum chromodynamics (QCD) unit of length	lQCD	Template:Val
Natural units based on the electronvolt	1 eV−1	Template:Val

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

Template:See also 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 = Template:Convert
• horse length ≈ Template:Convert

See also

• Template:Section link
• List of examples of lengths
• Template:Section link
• Medieval weights and measures
• Orders of magnitude (length)
• System of measurement
• Units of measurement

Further reading

• Template:Cite book