Editing Length (section)

== Units ==
{{Main|Unit of length}}
In the physical sciences and engineering, when one speaks of {{em|[[unit of length|units of length]]}}, the word {{em|length}} is synonymous with [[distance]]. There are several [[Units of measurement|units]] that are used to [[Measurement|measure]] length. Historically, units of length may have been derived from the lengths of human body parts, the distance travelled in a number of paces, the distance between landmarks or places on the Earth, or arbitrarily on the length of some common object.

In the [[International System of Units]] (SI), the [[SI base unit|base unit]] of length is the [[metre]] (symbol, m), now defined in terms of the [[speed of light]] (about 300 million metres per [[second]]). The [[millimetre]] (mm), [[centimetre]] (cm) and the [[kilometre]] (km), derived from the metre, are also commonly used units. In [[U.S. customary units]], English or [[imperial system of units]], commonly used units of length are the [[inch]] (in), the [[foot (length)|foot]] (ft), the [[yard]] (yd), and the [[statute mile|mile]] (mi). A unit of length used in [[navigation]] is the [[nautical mile]] (nmi).<ref>{{cite book|last=Cardarelli|first=François|title=Encyclopaedia of Scientific Units, Weights, and Measures: Their SI Equivalences and Origins|url=https://archive.org/details/encyclopaediaofs0000card|url-access=registration|year=2003|publisher=Springer|isbn=9781852336820 }}</ref>

{{calculator|id=km|type=number|size=9|default=1.609344|formula=miles*1.609344}} km = 
{{calculator|id=miles|type=number|size=16|default=1|formula=km/1.609344}} miles 

Units used to denote distances in the vastness of space, as in [[astronomy]], are much longer than those typically used on Earth (metre or kilometre) and include the [[astronomical unit]] (au), the [[light-year]], and the [[parsec]] (pc).

Units used to denote sub-atomic distances, as in [[nuclear physics]], are much smaller than the millimetre. Examples include the [[fermi (unit)|fermi]] (fm).