Unit of length

Many different units of length have been used around the world. The main units in modern use are U.S. customary units in the United States and the Metric system 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

Main article: Metric system

SI

Main article: International System of Units

See also: Orders of magnitude (length)

The 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 1/299,792,458 of a second." It is approximately equal to 1.0936 yards. Other units are derived from the metre by adding prefixes from the table below:

Standard prefixes for the metric units of measure (multiples)

Prefix name	N/A	deca	hecto	kilo	mega	giga	tera	peta	exa	zetta	yotta	ronna	quetta
Prefix symbol		da	h	k	M	G	T	P	E	Z	Y	R	Q
Factor	100	101	102	103	106	109	1012	1015	1018	1021	1024	1027	1030

Standard prefixes for the metric units of measure (submultiples)

Prefix name	N/A	deci	centi	milli	micro	nano	pico	femto	atto	zepto	yocto	ronto	quecto
Prefix symbol		d	c	m	μ	n	p	f	a	z	y	r	q
Factor	100	10−1	10−2	10−3	10−6	10−9	10−12	10−15	10−18	10−21	10−24	10−27	10−30

For example, a kilometre is 1000 metres.

Non-SI

In the Centimetre–gram–second system of units, the basic unit of length is the centimetre, or 1/100 of a metre.

Non-SI units of length include:

• fermi (fm) (= 1 femtometre in SI units)
• ångström (Å) (= 100 picometres in SI units)
• micron (= 1 micrometre in SI units)
• Norwegian/Swedish mil/myriametre (= 10,000 metres)
• x unit (= 0.1 picometre)

Imperial/US

Main articles: Imperial unit and U.S. customary units

The basic unit of length in the Imperial and U.S. customary systems is the yard, defined as exactly 0.9144 m by international treaty in 1959.

Common Imperial units and U.S. customary units of length include:

• thou or mil (1/1000 of an inch)
• line (1/12 of an inch)
• inch (2.54 cm)
• foot (12 inches, 0.3048 m)
• yard (3 ft, 0.9144 m)
• (terrestrial) mile (5280 ft, 1609.344 m)
• (land) league (3 miles)

Marine

In addition, the following are used by mariners:

• fathom (for depth; only in non-metric countries) (2 yards = 1.8288 m)
• nautical mile (one minute of arc of latitude = 1852 m)

Aviation

Aviators use feet (same as US) for altitude worldwide except in Russia and China.

Surveying

Surveyors in the United States continue to use:

• chain (~20.1m)
• rod (also called pole or perch) (~5 m)

Science

Astronomy

Main article: Astronomical system of units

Astronomical measure uses:

• Earth radius ($R_{\oplus }$) (≈6,371 km)
• astronomical unit (au or ua) (defined in 2012 as 149,597,870,700 m) Approximately the distance between the Earth and Sun.
• light year (ly) (≈9,460,730,472,580.8 km) The distance that light travels in a vacuum in one Julian year.
• parsec (pc) (≈30,856,775,814,671.9 km or about 3.26156 ly)
• hubble length (13.8 billon light year/306593922 parsec)

Physics

See also: Natural units

• Atomic unit of length (Bohr radius): $a_{0}\approx 0.529\;177\;210\;92\times 10^{-10}{\mbox{ m}}$
• Natural unit of length: ${\bar {\lambda }}_{\text{C}}\approx 386.15926800\times 10^{-15}{\mbox{m}}$
• Planck length: $\ell _{\text{P}}\approx 1.616\;199\times 10^{-35}{\mbox{ m}}$

Archaic

Main article: Ancient weights and measures

Archaic units of distance include:

• cana
• cubit
• rope
• league
• li (China)
• pace (the "double pace" of about 5 feet used in Ancient Rome)
• verst (Russia)

See also English units of length.

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 (9.5–10.9 metres in length)
• Football field (generally around 110 metres, depending on the country)
• Thickness of a human hair (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, a jocular unit of length created as part of an MIT fraternity prank

Other

Horse racing and other equestrian activities keeps alive:

• furlong = ~ 0.125 miles (201 m)
• horse length = ~ 8 feet (2.4 m)

See also

• English unit
• Medieval weights and measures
• Orders of magnitude (length)
• Systems of measurement

Further reading

• Whitelaw, Ian (2007). A Measure of All Things: The Story of Man and Measurement. Macmillan. ISBN 9780312370268.