Length unit converter

One converter for every length unit, from the nanometre to the nautical mile.

Pick a source and a target and you also get the equivalent in the remaining units of the same quantity.

Every unit is expressed internally in metres using its official factor. The value is converted to metres first and then to the target unit, which avoids compounding rounding errors.

A metre is no longer a bar kept in Paris

For almost a century the metre was the length of a platinum-iridium prototype held at Sèvres. Since 1983 it has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second. The definition changed, the size did not: any laboratory can reproduce it without visiting a unique object.

All imperial units hang off the yard

Inch, foot, yard and mile are defined from the yard of exactly 0.9144 metres fixed in 1959. That means imperial units are, paradoxically, defined today in metric terms: twelve inches to a foot, three feet to a yard, 1,760 yards to a mile.

Units that survive by custom

The nautical mile of 1,852 metres equals one minute of arc of latitude, which makes it natural for navigation. The light year, almost ten trillion kilometres, measures distance and not time despite the name. The astronomical unit is the mean Earth-Sun distance.

At small scales

A micrometre is a thousandth of a millimetre and a nanometre a millionth. A human hair is around 70 micrometres thick and a virus measures tens of nanometres.