Unit of length

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

A Unit of length refers to any discrete, pre-established length or distance having a constant magnitude which is used as a reference or convention to express linear dimension. The most common units in modern use are U.S. customary units in the United States and metric units elsewhere. 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.

Metric system

Main article: Metric system

SI

Main article: International System of Units

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 1/299,792,458 of a second." It is approximately equal to 1.0936 yards. Other units are derived from the metre by adding prefixes from the table below:

Standard prefixes for the SI units of measure

Multiples	Prefix name		deca	hecto	kilo	mega	giga	tera	peta	exa	zetta	yotta
	Prefix symbol		da	h	k	M	G	T	P	E	Z	Y
	Factor	100	101	102	103	106	109	1012	1015	1018	1021	1024
Fractions	Prefix name		deci	centi	milli	micro	nano	pico	femto	atto	zepto	yocto
	Prefix symbol		d	c	m	μ	n	p	f	a	z	y
	Factor	100	10−1	10−2	10−3	10−6	10−9	10−12	10−15	10−18	10−21	10−24

For example, a kilometre is 1000 metres.

Non-SI

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

Name	Symbol	SI value
fermi	fm	femtometre
ångström	Å	100 picometres
micron	μm	1 micrometre
Norwegian/Swedish mil or myriametre		10,000 metres
x unit	xu	0.1 picometre

Imperial/US

Main articles: Imperial units and United States customary units

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.

Common Imperial units and U.S. customary units of length include:

• thou or mil (1/1000 of an inch)
• line (1/12 of an inch)
• inch (2.54 cm)
• foot (12 inches, 0.3048 m)
• yard (3 ft, 0.9144 m)
• (terrestrial) mile (5280 ft, 1609.344 m)
• (land) league (3 miles)

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

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

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 (~20.1m)
• rod (also called pole or perch) (~5 m)

Science

Astronomy

Main article: Astronomical system of units

Astronomical measure uses:

• Earth radius R_⊕ ≈ 6,371 km
• Lunar distance LD ≈ 384402 km average distance between the center of Earth and the center of the Moon.
• astronomical unit AU, au or ua. Defined as 149597870700 m. 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.
• 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
The Compton wavelength of the electron	λC	2.426310215(18)×10−12
The reduced Compton wavelength of the electron	λC	386.15926764(18)×10−15
The Compton wavelength (or reduced Compton wavelength) of any fundamental particle.	λx
The Bohr radius of the hydrogen atom (Atomic unit of length)	a0	5.291772083(19)×10−11
The reduced wavelength of hydrogen radiation	1 / R∞	9.112670505509(83)×10−8
The Planck length	𝓁P	1.616199(97)×10−35
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 electron-Volt	1 eV-1	1.97×10−7

Archaic

See also: English units of length

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 (9.5–10.9 metres in length)
• Football field (generally around 110 metres, depending on the country)
• Thickness of a human hair (around 80 micrometres)
• A beard-second is a unit created as a teaching concept. It is the distance that a beard grows in a second (about 5 nanometres)
• Smoot, a jocular unit of length created as part of an MIT fraternity prank

Other

Horse racing and other equestrian activities keep alive:

• furlong ≈ 0.125 miles (201 m)
• horse length ≈ 8 feet (2.4 m)

See also

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