Unit of length


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 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 is the metre, defined as "the length of the path travelled by light in vacuum during a time interval of seconds." It is approximately equal to. Other units are derived from the metre by adding prefixes from the table below:
For example, a kilometre is. A slang term for the kilometre in the US and UK militaries is .

Non-SI

In the Centimetre–gram–second system of units, the basic unit of length is the centimetre, or of a metre.
Other non-SI units are derived from decimal multiples of the metre.
NameSymbolSI value
fermifmfemtometre
ångströmÅ100 picometres
micronμm1 micrometre
Norwegian/Swedish mil or myriametre10,000 metres
x unitxu0.1 picometre

Imperial/US

The basic unit of length in the Imperial and U.S. customary systems is the yard, defined as exactly by international treaty in 1959.
Common Imperial units and U.S. customary units of length include:
In addition, the following are used by sailors:
use feet for altitude worldwide and nautical miles for distance.

Surveying

in the United States continue to use:

Astronomy

measure uses:
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 propertySymbolLength, in metresReference
The classical electron radiusre
The Compton wavelength of the electronλC
The reduced Compton wavelength of the electronC
The Compton wavelength of any fundamental particlex
The Bohr radius of the hydrogen atom a0
The reduced wavelength of hydrogen radiation1 / R∞
The Planck length?P
Stoney unit of lengthlS
Quantum chromodynamics unit of lengthlQCD
Natural units based on the electronvolt1 eV−1

Archaic

units of distance include:
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:
and other equestrian activities keep alive: