Units of Measurement/Length

Length
Dimension		L
Usual Symbol		l
Coherent units
System	Unit	Symbol
SI	metre	m
CGS	centimetre	cm
Imperial	foot	ft

SI units

The base unit of length in the SI system is the metre. This has had several definitions over time:

1889

The International Bureau of Weights and Measures defines the metre as the distance between two marks on a platinum-iridium alloy bar (the International Prototype Metre). This prototype is kept in a vault in Sèvres, near Paris.

1960

The 11th General Conference on Weights and Measures redefines the metre in terms of a specific number of wavelengths of light of a specific frequency in a vacuum. It is defined to be 1 650 763.73 wavelengths of the electromagnetic radiation emitted by an electron transitioning between the levels 2p₁₀ and 5d₅ of the krypton-86 (⁸⁶Kr) atom in a vacuum. The metre is now defined in terms of reproducible physical effects, rather than a synthetic artifact.

1983

The 17th General Conference on Weights and Measures redefines the metre in terms of the distance traversed by light (having a universally constant speed in a vacuum) in a specific time. It is defined as the distance traveled by a beam of light in a vacuum in 1/299 792 458 second. This is the current definition. This has the effect that the speed of light is defined to be exactly 299 792 458 m/s.

Conversion of metric units of length

A			B
Unit	Symbol		pm	nm	μm	mm	cm	m	km
picometre	pm		1	10-3	10-6	10-9	10-10	10-12	10-15
ångström†	Å		102	10−1	10−4	10−7	10−8	10−10	10−13
nanometre	nm		103	1	10−3	10−6	10−7	10−9	10−12
micrometre	μm		106	103	1	10−3	10−4	10−6	10−9
millimetre	mm		109	106	103	1	10−1	10−3	10−6
centimetre	cm		1010	107	104	101	1	10−2	10−5
metre	m		1012	109	106	103	102	1	103
kilometre	km		1015	1012	109	106	105	103	1
1 A unit = b B units (b given in the table) † Ångström is accepted for use with, but not defined by the SI system. Its use is officially discouraged by the CIPM.

Imperial units

One international yard is defined as 0.914 4 m