A megametre (American spelling: megameter) (symbol: Mm) is a unit of length equal to 10⁶ metres (from the Greek words megas = big and metro = count/measure). Its customary equivalent is 621.37 miles.

• The Earth's polar circumference is, by definition, 40.00 Mm
• The distance from Amsterdam to Bordeaux is approximately 1 Mm
• The Earth's equatorial diameter is 12.76 Mm
• The mean distance from the Earth to the Moon is 384.4 Mm
• Jupiter's equatorial diameter is 143 Mm.

Megametres are rarely seen in practical use, because they are too big for most terrestrial tasks. They can also be easily confused with millimetres (mm). Still there are a number of "megametre fans". Megametres are also occasionally found in science fiction.

See also: square megametre, cubic megametre, 1 E6 m, SI, metre, SI prefix, Orders of magnitude, light year, parsec

kilometre << megametre << gigametre

A megameter is also a type of astrometrical instrument used for determining longitude by observation of the stars.