Unit of length

# Unit of length

Discussion
 Ask a question about 'Unit of length' Start a new discussion about 'Unit of length' Answer questions from other users Full Discussion Forum

Encyclopedia
Many different units of length have been used across the world. The main units in modern use are U.S. customary units in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

and the Metric system
Metric system
The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...

elsewhere. British Imperial unit
Imperial unit
The system of imperial units or the imperial system is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The system came into official use across the British Empire...

s are still used for some purposes in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

and some other countries. The metric system is sub-divided into SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

and non-SI units.

### SI units

Common units of length in the International System of Units (SI) are:
• metre
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

and its multiples, such as "centimetre
Centimetre
A centimetre is a unit of length in the metric system, equal to one hundredth of a metre, which is the SI base unit of length. Centi is the SI prefix for a factor of . Hence a centimetre can be written as or — meaning or respectively...

" or "kilometre
Kilometre
The kilometre is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second...

"

### Non-SI units

Non-SI units of length include:
• fermi (fm) (= 1 femtometre
Femtometre
The femtometre is an SI unit of length equal to 10-15 metres. This distance can also be called fermi and was so named in honour of Enrico Fermi and is often encountered in nuclear physics as a characteristic of this scale...

in SI units)
• angstrom
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

(Å) (= 100 picometre
Picometre
A picometre is a unit of length in the metric system, equal to one trillionth, i.e. of a metre, which is the current SI base unit of length...

s in SI units)
• micron (= 1 micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

in SI units)
• Norwegian/Swedish mil
Norwegian/Swedish mil
A mil is a unit of distance, most often used to measure geographic distance, very common in Norway and Sweden. Today, it measures by definition 10 kilometres, but earlier in history it had different values....

(= 10,000 metres)

## Imperial/US units

Common Imperial units and U.S. customary units of length include:
• inch
Inch
An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...

(2.54 cm)
• mil (one thousandth of an inch, one thou
Thou (unit of length)
A thou also known as a mil or point, is the verbalized abbreviation for "thousandth of an inch." It is a unit of length equal to 0.001 inch....

)
• foot (12 inches, 0.3048 m)
• yard
Yard
A yard is a unit of length in several different systems including English units, Imperial units and United States customary units. It is equal to 3 feet or 36 inches...

(3 ft, 0.9144 m)
• (terrestrial) mile
Mile
A mile is a unit of length, most commonly 5,280 feet . The mile of 5,280 feet is sometimes called the statute mile or land mile to distinguish it from the nautical mile...

(5280 ft, 1609.344 m)

## Marine

In addition, the following are used by mariners:
• fathom
Fathom
A fathom is a unit of length in the imperial and the U.S. customary systems, used especially for measuring the depth of water.There are 2 yards in an imperial or U.S. fathom...

(for depth; only in non-metric countries) (2 yards = 1.8288 m)
• nautical mile
Nautical mile
The nautical mile is a unit of length that is about one minute of arc of latitude along any meridian, but is approximately one minute of arc of longitude only at the equator...

(one minute of arc of latitude = 1852 m)

## Surveying

Surveyors in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

continue to use:
• chain
Chain (unit)
A chain is a unit of length; it measures 66 feet or 22 yards or 100 links . There are 10 chains in a furlong, and 80 chains in one statute mile. An acre is the area of 10 square chains...

(~20.1m)
• rod
Rod (unit)
The rod is a unit of length equal to 5.5 yards, 5.0292 metres, 16.5 feet, or of a statute mile. A rod is the same length as a perch or a pole. In old English, the term lug is also used.-History:...

(also called pole or perch) (~5 m)

## Astronomical

Astronomical
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

measure uses:
Because the Earth is not perfectly spherical, no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km...

(RE) (~6,370 km)
• astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

(AU) (~150 gigametres)
• light year (ly) (~9.46 petametres)
• parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

(pc) (~30.8 petametres), including kiloparsec (kpc) and megaparsec (Mpc)

## Archaic units

Archaic units of distance include:
• cana
Cana (unit of length)
A cana was a unit of length used in the former Crown of Aragon. The exact meaning was not consistent but the use in Barcelona was a distance of 1.5708 metres....

• cubit
Cubit
The cubit is a traditional unit of length, based on the length of the forearm. Cubits of various lengths were employed in many parts of the world in Antiquity, in the Middle Ages and into Early Modern Times....

• Rope
Rope (unit)
A rope was a unit of measurement, used in Somersetshire in drainage, hedging, and wall building. It is both a unit of length and a unit of area. As a linear measure, used in drainage and hedging, it is equal to 20 feet, i.e. 6.096 m . As a measure of area, used in wall building, it is equal to...

• league
League (unit)
A league is a unit of length . It was long common in Europe and Latin America, but it is no longer an official unit in any nation. The league originally referred to the distance a person or a horse could walk in an hour...

• li
Li (unit)
The li is a traditional Chinese unit of distance, which has varied considerably over time but now has a standardized length of 500 meters or half a kilometer...

(China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

)
• pace (the "double pace" of about 5 feet used in Ancient Rome
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

)
• verst
Verst
A verst or werst is an obsolete Russian unit of length. It is defined as being 500 sazhen long, which makes a verst equal to 1.0668 kilometres ....

(Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

)

## Informal units

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
Double-decker bus
A double-decker bus is a bus that has two storeys or 'decks'. Global usage of this type of bus is more common in outer touring than in its intra-urban transportion role. Double-decker buses are also commonly found in certain parts of Europe, Asia, and former British colonies and protectorates...

(9.5–10.9 metres in length)
• Football
Football
Football may refer to one of a number of team sports which all involve, to varying degrees, kicking a ball with the foot to score a goal. The most popular of these sports worldwide is association football, more commonly known as just "football" or "soccer"...

field (generally around 110 metres, depending on the country)
• Widths of a human hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....

(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
Smoot
The smoot is a nonstandard unit of length created as part of an MIT fraternity prank. It is named after Oliver R. Smoot, a fraternity pledge to Lambda Chi Alpha, who in October 1958 lay on the Harvard Bridge , and was used by his fraternity brothers to measure the length of the bridge.-Unit...

, a jocular unit of length created as part of an MIT
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...

fraternity prank

## Other

Horse racing
Horse racing
Horse racing is an equestrian sport that has a long history. Archaeological records indicate that horse racing occurred in ancient Babylon, Syria, and Egypt. Both chariot and mounted horse racing were events in the ancient Greek Olympics by 648 BC...

and other equestrian activities keeps alive:
• furlong
Furlong
A furlong is a measure of distance in imperial units and U.S. customary units equal to one-eighth of a mile, equivalent to 220 yards, 660 feet, 40 rods, or 10 chains. The exact value of the furlong varies slightly among English-speaking countries....

= ~ 0.125 miles (201.2 m)
• horse length
Length (horse racing)
A horse length, or simply length, is a unit of measurement that refers to the length of a horse from nose to tail, approximately 8 feet, It is commonly used in Thoroughbred horse racing, where it describes the distance between horses in a race...

= ~ 8 feet (2.4 m)

Physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

also uses:
• Planck length
The Bohr radius is a physical constant, approximately equal to the most probable distance between the proton and electron in a hydrogen atom in its ground state. It is named after Niels Bohr, due to its role in the Bohr model of an atom...