Convert between metric and imperial length units instantly.
Length conversion is exact rather than approximate, because the imperial and metric systems were formally reconciled. Since the international yard and pound agreement of 1959, an inch is defined as exactly 25.4 millimetres. Everything else follows: a foot is 0.3048 metres, a yard 0.9144, and a mile exactly 1,609.344 metres. These are definitions, not measurements, so conversions can be as precise as you need.
The metre itself is defined by the speed of light — the distance light travels in vacuum in 1/299,792,458 of a second — which is why it no longer depends on any physical artefact. Two practical cautions. A nautical mile is a different unit again, 1,852 metres, originally one minute of latitude, and is used in aviation and at sea rather than on land. And the US survey foot differed very slightly from the international foot and was used in American land surveying for decades before being retired, which occasionally explains small discrepancies in older property records.
Exactly 25.4. This is a definition agreed internationally in 1959, not an approximation, which is why inch-to-millimetre conversions can be made exactly.
As the distance light travels in a vacuum in 1/299,792,458 of a second. Tying it to a physical constant removed the dependence on a metal bar kept as a standard.
It is a separate unit of 1,852 metres, originally one minute of latitude along a meridian. That link to navigation is why it persists in aviation and maritime use.
A slightly different foot, larger than the international foot by about two parts per million, used historically in American land surveying. It has been retired, but it explains minor discrepancies in older survey records.