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

SI

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

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

Science

Astronomy

Astronomical measure uses:

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

Archaic units of distance include:

Informal

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: