About the Length Converter
This length converter moves any distance between ten units spanning both measurement systems: millimetres, centimetres, metres, kilometres and micrometres on the metric side, and inches, feet, yards, miles and nautical miles on the other. Every conversion is routed through metres using exact internationally agreed factors, so an inch is treated as precisely 25.4 millimetres rather than a rounded approximation. Alongside the answer you get the per-unit factor written as a formula and a table showing your value expressed in all ten units at once, which is useful when you are cross-checking a drawing or a spec sheet.
How to use the Length Converter
- 1
Type the number you want to convert into the Value box.
- 2
Set From to the unit your number is currently in.
- 3
Set To to the unit you want it in.
- 4
Copy the result, or read the table underneath to see the same distance in all ten units simultaneously.
What people use it for
Reading American plans in metric
A ceiling height given as 8 ft 6 in becomes 2.5908 m. Convert the inches separately and add, or enter 102 inches directly for the same figure.
Running and cycling distances
A 10 km race is 6.2137 miles and a half marathon of 13.1 miles is 21.08 km. Useful when a training plan and a race entry use different systems.
Manufacturing and 3D printing tolerances
Layer heights and machining tolerances are quoted in microns. A 0.2 mm layer is 200 microns, and a 40 micron tolerance is 0.04 mm.
What a metre actually is
For most of its life the metre was a physical thing. Defined in 1793 as one ten-millionth of the distance from the equator to the North Pole along the meridian through Paris, it was later embodied in a platinum-iridium bar kept near Paris. That ended in 1983, when the General Conference on Weights and Measures redefined the metre as the distance light travels in a vacuum in 1/299,792,458 of a second. The speed of light was thereby fixed by definition at exactly 299,792,458 metres per second, and it is no longer something we measure. Since the second itself is defined by 9,192,631,770 cycles of a specific caesium-133 hyperfine transition, the metre is now reproducible in any properly equipped laboratory on Earth without reference to an artefact. The 2019 revision of the SI extended that same philosophy to the whole system, defining every base unit from fixed fundamental constants. Practically, this means the conversion factors in this tool are exact by definition rather than measured, which is why they carry so many decimal places.
The imperial factors, and why they are exact
Until 1959 the British and American inch differed very slightly, which caused genuine problems in precision engineering and aerospace. The International Yard and Pound Agreement of that year settled it: the yard was defined as exactly 0.9144 metres. Every other imperial length follows from that single number. An inch is a thirty-sixth of a yard, so exactly 25.4 mm; a foot is a third of a yard, so exactly 0.3048 m; and a statute mile is 1,760 yards, so exactly 1,609.344 m. The nautical mile sits outside that family entirely. It was originally one minute of arc of latitude, and was standardised in 1929 at exactly 1,852 metres, which is why it remains the unit of choice in aviation and shipping, where a chart distance maps directly onto degrees of latitude. For quick mental work, kilometres to miles is close to multiplying by 0.6214, and the Fibonacci coincidence is handy: consecutive Fibonacci numbers approximate the ratio, so 5 km is about 3 miles and 8 km is about 5 miles.
Tips
- Convert compound imperial measurements to a single unit first — turn 5 ft 7 in into 67 inches before converting.
- A micron is one thousandth of a millimetre; human hair runs roughly 50 to 100 microns across.
- For area or volume, never reuse a length factor directly — you must square or cube it, which is what the area and volume converters do for you.