Length

The straight-line distance between two points is referred to as the length. These two points could be anything like two objects, two places, or two endpoints of the same object, etc. Length is one of the seven base quantities in a standard system of quantities called the International System of Quantities (IQS).

Units of Length in Different Unit Systems

A unit is defined as a fixed amount or number set as a standard to measure a single quantity. A unit system or a system of units of measurement is a collection of units and a set of rules to calculate various quantities in those units. Currently, the most popular and widely adopted systems of unit measurement are the International System of Units (SI), The British Imperial System, and the United States Customary Units.

International System of Units (SI)

The International System of Units, popularly known by its abbreviation SI, is the current standard of the metric system, a decimal-based measurement system. In this system, the base unit of length is a meter. All the other units are derived with reference to the meter. The SI unit system uses metric prefixes to derive other units of a quantity. Those derived units are decimal multiples of each other. For example, to measure longer distances, it is common to use kilometer instead of meter. The metric prefix kilo is a factor of 1000, which means 1 kilometer is equivalent to 1000 meters. The SI unit system currently has 24 prefixes that are used to structure derived units.

Conversion of base unit meter to other derived units using SI prefixes

The Imperial System and United States Customary Units

The British Imperial System, popularly referred to as the Imperial System, is a widely used system of measurement in the United Kingdom. It was initially defined in the Weights and Measures Act of 1824. The last amendment to the system was in the Weights and Measures Act of 1985.

The unit of measurement of length in the Imperial System is the yard, which is defined in reference to the meter. As per the latest definition of yard in the Imperial System, the yard is equal to 0.9144 meters. The other Imperial units are defined with reference to the yard.

The United States Customary Units system is another system of measurement mainly used in the United States. This system is similar to the Imperial System of Units, except for some minor differences. Inch, foot, yard, mile, and league are the most commonly used length units in this system.

Both systems are based on earlier English units.

The most widely used length units in the Imperial System and United States Customary Units

Unit	Symbol	Conversion to other units	SI equivalent value
Inch	in	1/36 yd or 1/12 ft	25.4 mm
Foot	ft or ‘	12 in	0.3048 m
Yard	yd	3 ft	0.9144 m
Mile	mi	5280 ft or 1760 yd	1.609344 km

Length units commonly used in Survey measures

Unit	Symbol	Conversion to other units	SI equivalent value
Link	li	33/50 ft or 7.92 in	0.201168 m
Rod or Pole or Perch	rd	25 li or 16.5 ft	5.0292 m
Chain	ch	4 rd or 66 ft	20.1168 m
Furlong	fur	10 ch	201.168 m
League	lea	3 mi or 15840 ft	4.828032 km
US Survey Foot	ft or ‘		1200/3937 m or 0.3048006 m
US Survey Mile	mi	8 fur	1.609344 km (2023 onwards)

International maritime units of distance

Unit	Symbol	Conversion to other units	SI equivalent value
Fathom	ftm	2 yd or 6 ft	1143/625 m or 1.8288 m
Cable	cb	120 ftm or 1.091 fur	3428/15625 km or 0.219456 km
Nautical mile	Nmi or NM	1.151 mi or 8.439 cb	1852 m or 1.852 km
Nautical league	nl or NL	3 NM	5556 m

Other less commonly used Imperial and US Customary units adopted from earlier English Units

Unit	Symbol	Conversion to other units	SI equivalent value
Thou	th or mil	1/1000 in	0.0254 mm
Barleycorn	barleycorn	1/3 in	0.00846667 m*
Nail (cloth)	nail	1/16 yd	0.05715 m
Hand	hand	4 in	0.1016 m
Shaftment	shaftment	6 in	0.1524 m
Span	span	9 in	22.86 cm
Ell	ell	45 in	1.143 m

Typography Units

Unit	Symbol	Conversion to other units	SI equivalent value
Twip	twip	1/20 p or 1/1440 in	17.6388888 µm*
Point	p	1/72 in	127/360 mm or 352.778 μm*
Pica	P	12 p	127/30 mm or 4.233 mm*

* approx. values