What next?
Length usually feeds an area or volume figure.
The unit mix-up that crashed a Mars orbiter
In 1999 NASA lost a $125 million spacecraft because one team used metric and another used imperial units. Length conversions aren’t trivia — getting them wrong has real consequences, on Earth and beyond.
Whether you’re reading a recipe, a blueprint or an overseas product spec, converting cleanly between metric and imperial keeps you on the right side of costly mistakes.
Common conversions
How to use this converter
- Type the value you want to convert in the “From” box.
- Pick the unit you’re converting from and the unit you want.
- Use the ⇄ swap button to reverse the direction instantly.
- The converted result and full equation update as you type.
Worked example
Converting 5 kilometres into miles:
5 km = 5,000 m
5,000 ÷ 1,609.344 = 3.107 miles
Going the other way, a 26.2-mile marathon is 42.195 km — the standard distance is defined in kilometres, and the mile figure is the rounded one.
What your result means
The converted length is exact by definition, not by measurement. Since 1959 the international inch has been fixed at precisely 25.4 mm, so every imperial-to-metric length conversion is an exact arithmetic step — the only error is whatever was already in the number you typed.
That matters for how you round. If you measured a shelf as "48 inches" with a tape marked in quarter-inches, the honest answer is about 1.22 m, not 1.2192 m. Carry the extra digits through intermediate steps if you are chaining conversions, then round once at the end to the precision you actually measured to.
Why this one is different
A quick-reference table is generated from whichever pair you picked, so common values are there to copy without re-typing eight conversions. The factor line states the ratio both ways round, and for imperial-to-metric it is exact rather than approximate: a mile is defined as 1,609.344 metres, not measured as it.
Exact by definition, and why that matters
Since 1959 the international yard has been defined as exactly 0.9144 metres, which makes the inch exactly 25.4 mm and every imperial length conversion exact rather than approximate. This is unusual: most unit conversions between systems are measured values with an uncertainty. Here the numbers are definitions, so a converted length carries no error at all beyond the rounding you choose to display.
The exception worth knowing about is the US survey foot, which was defined as 1200/3937 metres and differs from the international foot by about 2 parts per million. Irrelevant across a room, it amounts to roughly 3 mm per kilometre — enough to matter in land surveying, which is why the US formally retired it at the end of 2022 in favour of the international definition.
For practical work the more common failure is not the conversion but the mixing. A drawing that carries some dimensions in millimetres and some in inches will eventually be built wrong, regardless of how exact each individual conversion was. Pick the unit the work will be executed in, convert once at the start, and keep the whole drawing in it.
Frequently asked questions
How is a mile defined?＋
An international mile is exactly 1,609.344 metres, or 5,280 feet. This is what the converter uses.
Are results rounded?＋
The display is rounded for readability, but the calculation uses full precision internally.
How exact are inches and feet?＋
Exactly. Since 1959 the international inch has been defined as 25.4 mm, so inch, foot, yard and mile conversions are exact by definition rather than approximate.
What is the difference between a nautical mile and a statute mile?＋
A nautical mile is 1,852 metres, one minute of latitude, and is used at sea and in the air. A statute mile is 1,609.344 metres.
How do I convert a fractional inch?＋
Convert the fraction to a decimal first: 3/8 inch is 0.375 inch, which is 9.525 mm. Tape measures marked in sixteenths make this the most common workshop conversion.
Which unit should a drawing use?＋
Pick one and stay in it for the whole drawing. Mixed-unit drawings are the usual source of build errors, and millimetres avoid decimals for most construction work.
Related converters
Further reading
Assumptions & limitations
Length conversions are exact by definition, but a few things are worth knowing:
- The inch, foot, yard and mile used here are the international definitions (1 inch = 25.4 mm exactly), agreed in 1959.
- The US survey foot differs from the international foot by 2 parts per million. It matters in land surveying and nowhere else — it was officially retired at the end of 2022.
- A nautical mile is 1,852 m exactly and is not related to the statute mile.
- Displayed results are rounded for readability; the calculation itself carries full precision.