The inch is exactly 25.4 mm, and the agreement is younger than most houses
2 min read · figures checked · How we check
Before 1959 the American inch and the British inch were different lengths. The number that settled it is exact by definition, not by measurement.
One inch is exactly 25.4 millimetres, fixed by the international yard and pound agreement of 1959. The figure is a definition: it has no error bar and no more decimal places. Every figure here is computed by the same evaluator the calculators use, from the same sourced specification, rather than researched separately for the article. Where a number is exact by definition the article says so; where it is a measurement or a convention, it says that instead.
What changed in 1959
Until then, English-speaking countries each maintained their own physical standard for the yard, and the copies had drifted apart. The American yard and the imperial yard differed by roughly two parts per million — invisible on a tape measure, and not invisible at all when machining parts on two continents that have to fit.
The 1959 agreement replaced the physical standards with a definition: one yard is exactly 0.9144 metres. Everything else follows by division. A foot is exactly 0.3048 metres, an inch exactly 25.4 millimetres, a mile exactly 1609.344 metres.
Exact means exact
A conversion factor that comes from a definition behaves differently from one that comes from a measurement. There is no uncertainty to propagate and no point at which more decimal places would be more accurate — 25.4 is the whole of it. Any table that gives the inch as 25.400051 is quoting the pre-1959 US survey inch, which still exists in old land records and nowhere else.
| Unit | Exactly |
|---|---|
| 1 inch | 25.4 mm |
| 1 foot | 0.3048 m |
| 1 yard | 0.9144 m |
| 1 mile | 1609.344 m |
| 1 pound | 0.45359237 kg |
The one that did not go away
The US survey foot, defined as 1200/3937 metres, stayed in use for land surveying because rewriting a century of plats was worse than keeping a second foot. It is about two parts per million longer than the international foot: negligible across a room, around 3 mm across a mile, and enough to matter at the scale of a state plane coordinate system. It was formally retired at the end of 2022.
Every length conversion on this site uses the international definition. Where a figure is exact the page says so, because "exactly 2.54 cm" and "about 2.54 cm" are different claims and should not look alike.
When two parts per million matter
Almost never, and then suddenly. Across a 12-foot room the difference between the two feet is about seven thousandths of a millimetre — less than the width of the pencil line you marked with. Across a mile it is roughly 3 mm. Across a state plane coordinate grid, where coordinates run to hundreds of thousands of feet, it is metres, and it puts a boundary in the wrong place.
That is the whole reason the survey foot outlived the agreement that replaced it: the error is invisible at the scale where people build and unmissable at the scale where people record who owns what.
A related trap on a tape measure: the imperial side is divided into halves of halves, so the marks are 1/2, 1/4, 1/8, 1/16. There is no 1/10, and reading "point one inch" off a fraction tape is a conversion, not a reading.
It is also why a tape with both scales is worth owning. Reading 43.7 cm off the metric edge and 17-3/16 off the imperial one gives the same length twice, and the agreement between them is a check on the reading. Converting a fraction in your head on a job site is where the error enters, not the tape.
Do the arithmetic
- Inches to centimetresWith the factor and its source
- Feet to metresExactly 0.3048, by the same agreement
- Feet and inches calculatorAdd and subtract in feet-and-inches