Length Converter
Convert between length units like meters, kilometers, miles.
Length conversion and the definitions behind it
Every length unit in use today is defined against the metre, and the metre itself is defined by the speed of light. That means most conversion factors are exact by definition rather than measured, which is why the inch is precisely 25.4 mm with no rounding.
Formula
target = source x (source factor to metres / target factor to metres)
Converting through the metre as a pivot avoids accumulating rounding error across chained conversions.
Where the metre comes from
The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole. That definition was replaced by a platinum-iridium bar, then in 1960 by a wavelength of krypton-86 radiation.
Since 1983 the metre has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second. The speed of light is therefore not measured - it is fixed by definition, and the metre follows from it.
The 2019 SI redefinition tied every base unit to a fixed physical constant, so no artefact anywhere is the reference for any unit. The kilogram prototype in Sevres is now a historical object rather than a standard.
Exact factors and the ones that are not
The 1959 International Yard and Pound Agreement fixed the imperial units against the metre exactly, ending a long period where the US and UK inch differed slightly.
- 1 inch = 25.4 mm exactly. 1 foot = 0.3048 m exactly. 1 yard = 0.9144 m exactly. 1 mile = 1,609.344 m exactly.
- 1 nautical mile = 1,852 m exactly, defined as one minute of latitude at the equator and adopted internationally in 1929.
- The US survey foot differed from the international foot by 2 parts per million and was officially retired at the end of 2022, though it persists in older land records.
- Astronomical units are not exact in the same way: a light-year is 9.4607 x 10^15 m and a parsec is 3.0857 x 10^16 m, both derived rather than defined.
Where conversion errors do real damage
The 1999 Mars Climate Orbiter was lost because one team supplied thrust data in pound-force seconds while the receiving software expected newton-seconds. The 125 million dollar spacecraft entered the atmosphere at the wrong altitude and broke up.
The 1983 Gimli Glider incident saw an Air Canada 767 run out of fuel at 41,000 feet after a conversion between pounds and kilograms was performed with the wrong factor during Canada's metric transition.
The common thread is not arithmetic difficulty. It is an unstated assumption about which unit a number was already in. Labelling units on every value, in code and on paper, prevents almost all of it.
Worked example
Convert 6 feet 2 inches to centimetres. Total inches: (6 x 12) + 2 = 74. Then 74 x 2.54 = 187.96 cm.
Going the other way, 180 cm to feet and inches: 180 / 2.54 = 70.866 inches. Divide by 12 to get 5 feet with 10.866 inches remaining, so 5 feet 10.9 inches, or 5 feet 11 inches when rounded.
Note the asymmetry in precision: 5 feet 11 inches is 180.34 cm, not 180. Rounding to the nearest inch loses up to 1.27 cm, which matters for anything being cut or fitted.
Length units in metres
| Unit | In metres | Exact? |
|---|---|---|
| millimetre | 0.001 | Yes |
| centimetre | 0.01 | Yes |
| inch | 0.0254 | Yes, by 1959 agreement |
| foot | 0.3048 | Yes |
| yard | 0.9144 | Yes |
| fathom | 1.8288 | Yes (6 feet) |
| kilometre | 1,000 | Yes |
| mile | 1,609.344 | Yes |
| nautical mile | 1,852 | Yes, by definition |
Frequently Asked Questions
Related calculators
References
- BIPM - The International System of Units (SI Brochure, 9th edition)
- NIST - US survey foot deprecation notice
Last reviewed: 2026-08-07. This page is informational. For legal, medical, tax, or financial decisions, confirm the result with a qualified professional.