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Length converter

Convert millimetres, centimetres, metres, kilometres, inches, feet, yards and miles, with the feet and inches reading worked out for heights and room measurements.

The length you have

Every imperial figure here is exact rather than rounded. An inch is defined as 25.4 mm, so a mile is exactly 1,609.344 m.

Converted

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Enter a length and pick the two units.

These conversions are exact, not approximate

Most conversion tables give you a rounded figure and leave you guessing how much was rounded away. That is not necessary here, because the imperial units are defined in metric ones rather than measured against them.

The international yard and pound agreement of 1959 set the inch at exactly 25.4 millimetres. Everything else follows from that by whole number multiplication:

  • 1 foot = 12 inches = exactly 304.8 mm
  • 1 yard = 3 feet = exactly 0.9144 m
  • 1 mile = 1,760 yards = exactly 1,609.344 m

So when this tool says a marathon is 42.1648128 km, every one of those digits is real. The 42.195 km you usually see is the distance as the race is actually defined, in metric, which is a slightly different number for a slightly different reason.

Heights, where the decimal answer is the wrong answer

Ask anyone how tall they are in feet and they will not say 5.9055. They will say five foot eleven. The decimal is correct arithmetic and useless conversation, so this tool shows both: the plain conversion, and the same length as whole feet plus the inches left over.

A few worth knowing by heart: 5′0″ is 152.4 cm, 5′6″ is 167.6 cm, 6′0″ is 182.9 cm. Every inch is 2.54 cm, so you can step between them in your head.

Which unit to use

Millimetres are the easiest to convert by hand, because the inch is defined in them. Half an inch is 12.7 mm, a quarter is 6.35 mm, a sixteenth is 1.5875 mm. Anyone working from a set of plans or a cutting list is usually better off in millimetres than in centimetres.

Centimetres are for bodies and furniture. Metres are for rooms and gardens. Kilometres and miles are for journeys, and the rough conversion worth carrying is that five miles is about eight kilometres, which is accurate to within half a percent.

Where the two systems still disagree

Since 1959 an inch has been an inch everywhere, so the United States and the United Kingdom no longer measure length differently. What is left is habit. The UK signs its roads in miles and sells petrol in litres. The US uses feet and inches almost everywhere outside science and medicine.

One genuine exception survived until recently. American surveyors kept a slightly longer foot, the US survey foot, which was larger by two parts in a million. Over a mile that is about three millimetres, which matters for a property boundary and for nothing else. It was retired at the end of 2022, and this tool uses the international foot.

Questions

How many centimetres are in an inch?
Exactly 2.54. That is not a rounded measurement, it is the definition: the international yard and pound agreement of 1959 set the inch at exactly 25.4 millimetres, and every other imperial length is built from it. So a foot is exactly 30.48 cm, a yard is exactly 0.9144 m and a mile is exactly 1,609.344 m.
How do I convert my height to feet and inches?
Enter your height in centimetres and read the 'in feet and inches' line. The plain conversion gives a decimal number of feet, which is arithmetically right and not how anyone says a height: 180 cm is 5.9055 feet, which people say as 5 foot 11. The tool works out both.
Is a mile really 1.609 kilometres?
It is exactly 1.609344 kilometres. A mile is 1,760 yards, a yard is exactly 0.9144 metres, and multiplying those gives 1,609.344 metres with nothing rounded away. The 1.609 you usually see is that figure cut short, which is close enough for a road sign and not for a survey.
What is the difference between a metre and a yard?
A metre is about 9.4% longer. A yard is 0.9144 m, so 100 yards is 91.44 m. On a sports field the difference is large enough to matter: a 100 yard American football field is roughly 8.5 metres shorter than a 100 metre running track.
Why do the United States and the United Kingdom measure differently?
They mostly do not any more, for length. Both countries signed the 1959 agreement, which gave the inch, foot, yard and mile identical definitions everywhere. What differs is usage: the UK signs roads in miles but sells cloth in metres, and the US uses feet and inches for almost everything outside science and medicine.
What was the US survey foot?
A slightly longer foot the United States kept for land surveying after 1959, larger by two parts in a million. Over a mile that is about 3 mm, and over a hundred miles it is about a foot, which is why surveyors needed it and nobody else did. It was retired at the end of 2022. This tool uses the international foot.
How many millimetres are in an inch?
Exactly 25.4. Millimetres are usually the easiest unit to work in when you are converting by hand, because the inch is defined in them: half an inch is 12.7 mm, a quarter inch is 6.35 mm and a sixteenth is 1.5875 mm.
Does anything I type here get sent anywhere?
No. The conversion runs in your browser, in the page you are already looking at. Nothing is uploaded, stored or logged, and closing the tab clears it. You can disconnect from the internet and the tool will keep working.

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