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.

Metric system

SI

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 1/undefined seconds." It is approximately equal to 1.094 yards. 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 1,000 meters.

Non-SI

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

Imperial/U.S.

The basic unit of length in the imperial and U.S. customary systems is the yard, defined as exactly 0.914 m by international treaty in 1959. Common imperial units and U.S. customary units of length include:

Marine

In addition, the following are used by sailors:

Aviation

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

Surveying

Surveyors in the United States continue to use:

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.

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 66 ft 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:

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:

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:

Other

Horse racing and other equestrian activities keep alive: