Length converter
Ten units of length converted at once: millimetres, centimetres, metres, kilometres, inches, feet, yards, miles, nautical miles and thou. The inch, foot, yard and mile are exact multiples of the metre by definition, so none of these conversions is an approximation.
How to convert length
Since 1959 the imperial units have been defined in terms of the metre rather than the other way round. An inch is exactly 25.4 millimetres, and the foot, yard and mile all follow from it: 0.3048 m, 0.9144 m and 1609.344 m, every one of them exact. This is why a conversion here never carries a rounding error of its own. Any error you see comes from the number you typed, or from the display, which shows four decimal places for values between 1 and 100 and two above that.
The nautical mile is the odd one out. It is 1,852 metres exactly, a round metric number fixed by the first Extraordinary International Hydrographic Conference in 1929 to sit as close as possible to one minute of latitude. That relationship is what makes it useful at sea: a degree of latitude is sixty of them, so a distance read off a chart converts to an angle without arithmetic. It has no such convenience anywhere else, which is why aviation and shipping use it and nobody else does.
Thou is the engineering unit that surprises people who have not met it. One thou is a thousandth of an inch, 25.4 micrometres, and it is how machining tolerances, feeler gauges, shim stock and paint film thickness are still specified in workshops that otherwise work in millimetres. American machinists usually call it a mil, which collides badly with the millimetre and with the millilitre, so thou is the safer word in writing.
Two things are deliberately absent. There is no US survey foot: it differed from the international foot by two parts per million, mattered only over the distances of a land survey, and was formally retired at the end of 2022. And there is no micrometre or nanometre row, so anything at that scale has to be entered as a fraction of a millimetre; the display switches to exponential notation below a millionth of a metre so the answer does not collapse to zero.
What people use it for
- Reading a distance off a chart in nautical miles
- Checking a material thickness quoted in thou
- Putting one measurement into several units for a single document
- Comparing a metric and an imperial spec sheet line by line
- Working out a pool or track distance in both yards and metres
- Converting a machining tolerance between thou and hundredths of a millimetre
Questions
Exactly 2.54, by definition since 1959. The definition runs from the metre to the inch, not the other way round.
Exact. Every imperial unit here is defined as a fixed number of metres, so the only rounding is in how many decimal places are printed.
Six below 1, four between 1 and 100, two above that, and exponential notation past a billion or below a millionth. Small conversions keep their precision instead of rounding to zero.
It is 1,852 metres, set to approximate one minute of latitude. That makes it natural for navigation and arbitrary anywhere else.
Sixty, near enough for chart work. The unit was defined to make that true.
A thousandth of an inch, 25.4 micrometres. Still standard for machining tolerances, shim stock and coating thickness.
Yes, in engineering. Avoid "mil" in writing though: it reads as millimetre to most of the world and as millilitre to some.
5,280, which is 1,760 yards. The number comes from the Roman mille passus by way of an Elizabethan statute that fixed the mile at eight furlongs.
No. It was two parts per million longer than the international foot and was officially retired at the end of 2022. Older US land records still use it.
Not directly; there is no row for either. Enter the value as a fraction of a millimetre, and the display keeps enough places to stay useful.
They are astronomical distances rather than lengths anyone measures, and mixing them into a table with millimetres makes every other row unreadable.
A yard is 0.9144 m, so about 8.5 per cent shorter. Over 100 units the gap is more than eight metres, which is why swimming times in yards and metres are not comparable.
It was standardised from the furlong, which came from ploughing, rather than from the foot. Eight furlongs of 660 feet gives 5,280.
Yes, and the sign carries through. Length is rarely negative, but a difference between two measurements often is.
No. The whole table is one multiplication per row and it runs in this page.