Length Converter

Since 1959 the inch has been defined as exactly 25.4 millimetres, which makes every imperial length conversion exact rather than approximate.

The exact conversion factors

These are definitions, not measurements, so they are exact:

The International Yard and Pound Agreement of 1959 settled this between the United States, United Kingdom, Canada, Australia, New Zealand and South Africa. Before it, the US and imperial inches differed very slightly — enough to matter in surveying, which is why the US survey foot persisted for land measurement until it was officially retired at the end of 2022.

Useful mental conversions

• Miles to kilometres: multiply by 1.6, or use consecutive Fibonacci numbers — 5 miles ≈ 8 km, 8 miles ≈ 13 km, 13 miles ≈ 21 km. The coincidence works because the golden ratio (1.618) is close to 1.609.
• Kilometres to miles: multiply by 0.6, or halve and add 10 %.
• Centimetres to inches: divide by 2.5 and subtract a little. 100 cm ≈ 39.4 in.
• Metres to feet: multiply by 3.3. A 100 m sprint is about 328 feet.
• Height: 6 feet is 183 cm; 5 ft 10 in is 178 cm.

Where the metre comes from

The metre was originally defined in 1793 as one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris — a definition requiring an expedition to survey. It was later embodied in a platinum-iridium bar kept near Paris, which had the disadvantage that the world's standard of length could be dropped.

Since 1983 it has been defined by the speed of light: the metre is the distance light travels in vacuum in 1/299,792,458 of a second. This makes the speed of light exact by definition and ties length to a universal constant that any laboratory can reproduce.

The nautical mile has a different logic entirely: it is one minute of latitude, which is why it is convenient for navigation. One nautical mile per hour is one knot.