Length Converter
An inch has been exactly 25.4 mm since 1959.
Convert Length. An inch has been exactly 25.4 mm since 1959. Every convention shown, not just one.
Converted
1 inches
25.4 millimetres · 2.54 centimetres
The inch has been exactly 25.4 millimetres since the international yard and pound agreement of 1959, which also fixed the foot at 0.3048 m and the mile at 1,609.344 m. Before that the US and imperial inches differed very slightly, which mattered for surveying and almost nothing else. Every factor in this family is exact by international agreement rather than measured, so the only rounding is in the display.
How the Length Converter works
Length between metric and imperial units, all in one view. Every factor here is exact by international agreement rather than measured, so the only rounding happens at the display.
Also known as: inches to cm converter · feet to metres converter · miles to kilometres · how many mm in an inch
Exact by agreement since 1959
The inch is exactly 25.4 millimetres, the foot exactly 0.3048 metres and the mile exactly 1,609.344 — all fixed by the international yard and pound agreement of 1959. They are definitions rather than measurements.
Before that the US and imperial inches differed very slightly, which mattered for precision engineering and surveying and almost nothing else. The agreement removed a discrepancy that had persisted for decades.
A legacy US survey foot was retained for existing survey data, differing by about two parts per million. It was officially retired in 2022, and it survives in old records where the distinction genuinely matters over long distances.
Where the metre came from
Originally one ten-millionth of the distance from the equator to the North Pole through Paris — an attempt to derive a unit from the Earth rather than from a monarch's body.
It became a platinum bar, then a multiple of a krypton wavelength, and since 1983 the distance light travels in a vacuum in 1/299,792,458 of a second. Each redefinition improved reproducibility without changing the length.
That last definition fixes the speed of light exactly, by construction. The speed is no longer measured — it defines the metre, which is why it has no uncertainty attached to it.
The units that survive in specific trades
Even thoroughly metric countries keep imperial units in particular places. Pipe sizes, screen diagonals, tyre rim diameters and bicycle wheel sizes are inch-based almost everywhere, because the manufacturing standards were set once and never converted.
Nautical and aviation units are their own case. A nautical mile is exactly 1,852 metres because it approximates one minute of latitude, which makes chart navigation arithmetic straightforward — a genuine functional reason rather than inertia.
Horse heights in hands, land in acres and beer in pints all persist for the same reason: the unit is embedded in a trade's practice and the cost of change exceeds the benefit. Conversion tools exist because both systems continue to be used.
Where to go next
The Length Converter question rarely arrives on its own. These are the ones that usually come with it:
- Weight Converter — A US ton and an imperial ton differ by 12%.
- Area Converter — An acre was a day's ploughing, and it stuck.
- Speed Converter — A knot is one nautical mile an hour, exactly.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
How many centimetres in an inch?
Exactly 2.54, by definition since the international yard and pound agreement of 1959. It is a definition rather than a measurement.
How many metres in a mile?
Exactly 1,609.344. That follows from the yard being defined as exactly 0.9144 m and a mile being 1,760 yards.
What is a nautical mile?
Exactly 1,852 metres, chosen because it is close to one minute of latitude. That makes navigation arithmetic straightforward on a chart, which is why it survives.
Why was the inch redefined in 1959?
Because the US and imperial inches differed very slightly, which caused problems in precision engineering and surveying. The agreement fixed a single value for both.
What is a survey foot?
A slightly different US foot retained for legacy surveying data after 1959, differing by about two parts per million. It was officially retired in 2022 and it persists in old records.
How do I convert feet and inches to metres?
Convert the whole thing to inches first, then multiply by 0.0254. Five feet ten inches is 70 inches, which is 1.778 metres.
How is the metre defined?
As the distance light travels in a vacuum in 1/299,792,458 of a second. It was previously a platinum bar and before that a fraction of the Earth's meridian.
What is a hand, as in horse height?
Exactly four inches. Horse heights are written like 15.2 hands, which means fifteen hands and two inches rather than fifteen point two.
What is a furlong?
One eighth of a mile, 220 yards — originally the length of a furrow in a ploughed field. It survives in horse racing and in the definition of the acre.
How long is a light-year?
About 9.46 trillion kilometres — a distance, despite the name. A parsec is about 3.26 light-years and is the unit astronomers actually prefer.
What is a micron?
A micrometre, one millionth of a metre. The name is deprecated in SI and it persists widely in engineering and filtration.
Why do some countries still use imperial units?
Historical inertia and the cost of change. Even metric countries retain imperial in specific trades — pipe sizes, screen diagonals and tyre rim diameters are inch-based almost everywhere.
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