What it is
The Length Converter handles millimetres, centimetres, metres and kilometres alongside inches, feet, yards and miles.
One number worth memorising
An inch is exactly 2.54 centimetres. Not approximately: since 1959 the inch has been defined against the metre, so that figure is the definition rather than a measurement.
Everything else in the imperial system follows from it. A foot is 12 inches, so 30.48 cm. A yard is 3 feet, so 91.44 cm. A mile is 1,760 yards, so 1.609 km.
Reading a height in feet and inches
Written as 5'11", that means five feet and eleven inches: 5 × 30.48 + 11 × 2.54, which is about 180 cm. The notation trips people up because the marks look like quotation marks, and because the inches part is not a decimal.
Screens, running and construction
Screen sizes are always the diagonal, in inches, everywhere in the world. A 55 inch television measures 140 cm corner to corner, and that says nothing about its width until you also know the aspect ratio.
Running distances are the reverse: a marathon is defined as 42.195 km, which works out to an untidy 26.219 miles precisely because the metric figure came first.
Construction and engineering in metric countries work in millimetres rather than centimetres, because a drawing without a decimal point has fewer ways to go wrong on site.
How to use it
Set both units, type in either box and read the other. It runs entirely in your browser, with no limit and no account.