A ruler, depicting two customary units of length, the centimetre and the inch
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.  
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 seconds." 1⁄ 299792458 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 . klick 
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.
Diagram of English length units and their integer relations to each other.
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:
thou or mil ( of an inch) 1⁄ 1000
foot (12 inches, 0.3048 m)
yard (3 feet, 0.9144 m) (terrestrial)
mile (5280 feet, 1609.344 m) (land) league 3 miles (4,800 m)
In addition, the following are used by
fathom (for depth; only in non-metric countries) (2 yards = 1.8288 m) nautical mile (one minute of arc of latitude = )
Aviators use feet for altitude worldwide (except in Russia and China) and nautical miles for distance.
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 ) rod (also called pole or perch) (quarter of a chain, 5 yards, or ) 1⁄ 2
Astronomical 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:
Length, in metres
The classical electron radius
The Compton wavelength of the electron
The reduced Compton wavelength of the electron
The Compton wavelength (or reduced Compton wavelength) of any fundamental particle
Bohr radius of the hydrogen atom ( Atomic unit of length)
The reduced wavelength of hydrogen radiation
Stoney unit of length
Quantum chromodynamics (
QCD) unit of length
Natural units based on the electronvolt
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:
Horse racing and other equestrian activities keep alive:
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