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 Metric units are units based on the metre, gram or second and decimal (power of ten) multiples or sub-multiples of these. The most widely used examples are the units of the International System of Units (SI). By extension they include units of e ...

, used in every country globally. In the United States the

U.S. customary units United States customary units form a system of measurement units commonly used in the United States and U.S. territories since being standardized and adopted in 1832. The United States customary system (USCS or USC) developed from English units ...

are also in use. British

Imperial units The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed th ...

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 The metre (British spelling) or meter (American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its pref ...

, defined as "the length of the path travelled by

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

in vacuum during a time interval of seconds." It is approximately equal to . Other SI units are derived from the metre by adding

prefixes A prefix is an affix which is placed before the stem of a word. Adding it to the beginning of one word changes it into another word. For example, when the prefix ''un-'' is added to the word ''happy'', it creates the word ''unhappy''. Particu ...

, as in millimetre or kilometre, thus producing systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. For example, a kilometre is .

Non-SI

In the

centimetre–gram–second system of units The centimetre–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units ...

, the basic unit of length is the

centimetre 330px, Different lengths as in respect to the Electromagnetic spectrum, measured by the Metre and its deriveds scales. The Microwave are in-between 1 meter to 1 millimeter. A centimetre (international spelling) or centimeter (American spellin ...

, or of a metre. Other non-SI units are derived from decimal multiples of the metre.

Imperial/US

The basic unit of length in the imperial and U.S. customary systems is the

yard The yard (symbol: yd) is an English unit of length in both the British imperial and US customary systems of measurement equalling 3 feet or 36 inches. Since 1959 it has been by international agreement standardized as exactly ...

, defined as exactly by international treaty in 1959. Common imperial units and U.S. customary units of length include: *

thou The word ''thou'' is a second-person singular pronoun in English. It is now largely archaic, having been replaced in most contexts by the word '' you'', although it remains in use in parts of Northern England and in Scots (). ''Thou'' is the ...

or mil ( of an inch) *

inch Measuring tape with inches The inch (symbol: in or ″) is a unit of length in the British imperial and the United States customary systems of measurement. It is equal to yard or of a foot. Derived from the Roman uncia ("twelfth") ...

() * foot (12 inches, 0.3048 m) *

(3 feet, 0.9144 m) * (terrestrial)

mile The mile, sometimes the international mile or statute mile to distinguish it from other miles, is a British imperial unit and United States customary unit of distance; both are based on the older English unit of length equal to 5,280 English ...

(5280 feet, 1609.344 m) * (land)

league League or The League may refer to: Arts and entertainment * ''Leagues'' (band), an American rock band * ''The League'', an American sitcom broadcast on FX and FXX about fantasy football Sports * Sports league * Rugby league, full contact footba ...

Marine

In addition, the following are used by

sailor A sailor, seaman, mariner, or seafarer is a person who works aboard a watercraft as part of its crew, and may work in any one of a number of different fields that are related to the operation and maintenance of a ship. The profession of the s ...

s: * fathom (for depth; only in non-metric countries) (2 yards = 1.8288 m) * nautical mile (one minute of arc of latitude = )

Aviation

Aviators An aircraft pilot or aviator is a person who controls the flight of an aircraft by operating its directional flight controls. Some other aircrew members, such as navigators or flight engineers, are also considered aviators, because they ar ...

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

Surveying

Determination of the rute and the feet in Frankfurt

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 )

Science

Astronomy

Astronomical Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxi ...

measure uses: *

Earth radius Earth radius (denoted as ''R''🜨 or R_E) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid, the radius ranges from a maximum of nearly (equatorial radius, deno ...

≈ 6,371 km * Lunar distance ''LD'' ≈ . Average distance between the center of Earth and the center of the Moon. *

astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...

au. Defined as . Approximately the distance between the Earth and Sun. * light-year ly ≈ . The distance that light travels in a

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...

in one Julian year. *

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...

pc ≈ or about * 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 In physics, natural units are physical units of measurement in which only universal physical constants are used as defining constants, such that each of these constants acts as a coherent unit of a quantity. For example, the elementary charge ma ...

of length are included in this table:

Archaic

Archaic units of distance include: *

cana Cana of Galilee ( grc, Κανὰ τῆς Γαλιλαίας; ar, قانا الجليل , translit= Qana al-Jalil , lit=Qana of the Galilee) is the location of the Marriage at Cana, at which the miracle of turning water into wine took place in ...

cubit The cubit is an ancient unit of length based on the distance from the elbow to the tip of the middle finger. It was primarily associated with the Sumerians, Egyptians, and Israelites. The term ''cubit'' is found in the Bible regarding No ...

rope A rope is a group of yarns, plies, fibres, or strands that are twisted or braided together into a larger and stronger form. Ropes have tensile strength and so can be used for dragging and lifting. Rope is thicker and stronger than similar ...

* li (China) *

pace Pace or paces may refer to: Business *Pace (transit), a bus operator in the suburbs of Chicago, US * Pace Airlines, an American charter airline *Pace Foods, a maker of a popular brand of salsa sold in North America, owned by Campbell Soup Compan ...

(the "double pace" of about 5 feet used in

Ancient Rome In modern historiography, ancient Rome refers to Roman civilisation from the founding of the city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD. It encompasses the Roman Kingdom (753–509 B ...

) *

verst A verst (russian: верста, ) is an obsolete Russian unit of length defined as 500 sazhen. This makes a verst equal to . Plurals and variants In the English language, ''verst'' is singular with the normal plural ''versts''. In Russian, the no ...

(Russia)

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 A double-decker bus or double-deck bus is a bus that has two storeys or decks. They are used for mass transport in the United Kingdom, the United States, New Zealand, Europe, Asia and also in cities such as Sydney; the best-known example is the ...

(9.5–11 metres in length) * Football field (100 yards in length) *Thickness of a human

hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and f ...

(around 80 micrometres)

Other

Horse racing Horse racing is an equestrian performance sport, typically involving two or more horses ridden by jockeys (or sometimes driven without riders) over a set distance for competition. It is one of the most ancient of all sports, as its basic p ...

and other equestrian activities keep alive: *

furlong A furlong is a measure of distance in imperial units and United States customary units equal to one eighth of a mile, equivalent to 660 feet, 220 yards, 40 rods, 10 chains or approximately 201 metres. It is now mostly confined to use in hor ...

≈ *

horse length A horse length, or simply length, is a unit of measurement for the length of a horse from nose to tail, approximately . Use in horse racing The length is commonly used in Thoroughbred horse racing, where it describes the distance between horses i ...

≈