The
kilometre (International spelling as used by the
International Bureau of Weights and Measures;
SI symbol:
km; or ) or
kilometer (American spelling) is a
unit of
length in the
metric system, equal to one thousand
metres (
kilo- being the
SI prefix for ). It is now the measurement unit used officially for expressing distances between geographical places on land in most of the world; notable exceptions are the United States and the road network of the United Kingdom where the
statute mile is the official unit used.