About this tool
Convert any length between metric and imperial units — millimeters through kilometers, inches through miles — with results for every unit shown at once. Conversions use exact definitions, not rounded shortcuts.
Since 1959 the inch has been defined as exactly 25.4 mm, which anchors the whole imperial chain to the metric system: a foot is exactly 0.3048 m and a mile exactly 1,609.344 m. Mixing the systems carelessly has real consequences — NASA lost the $125 million Mars Climate Orbiter in 1999 to a pound-force/newton unit mix-up.
How to use it
- Enter a value.
- Choose its unit.
- Read the equivalent in all other units instantly.
Frequently asked questions
Is an inch exactly 2.54 cm?
Yes — by international agreement since 1959, the inch is defined as exactly 25.4 mm, so conversions are exact, not approximate.
How many feet are in a mile?
5,280 feet. The number comes from the old definition of a mile as eight furlongs of 660 feet each.
Which countries still use imperial lengths?
Primarily the United States; the UK uses miles on roads but metric nearly everywhere else. Most of the world is fully metric.