Inches to Cm Converter
Convert inches to centimetres and back, with a reference table for heights and screen sizes. An inch is exactly 2.54 cm, so results are exact.
| Inches | Centimeters | |
|---|---|---|
| 1 in | 2.54 cm | |
| 6 in | 15.24 cm | |
| 12 in | 30.48 cm | 1 foot |
| 24 in | 60.96 cm | 2 feet |
| 36 in | 91.44 cm | 3 feet / 1 yard |
| 60 in | 152.4 cm | 5 feet |
| 72 in | 182.88 cm | 6 feet |
Every whole inch survives a round trip. 611 of the first 1,000 whole centimetres do not.
An inch has been exactly 2.54 centimetres since 1959 — a definition agreed by treaty, not a measurement anyone refines. So the factor is exact and both directions ought to behave alike. They do not, and the reason is decimal rather than physical. Multiplying by 2.54 adds at most two decimal places: 7 inches is exactly 17.78 cm and stops. Dividing does not stop. 1/2.54 is 50/127, and 127 is prime and neither 2 nor 5, so the expansion repeats forever: 0.39370078740157480314… One direction terminates and the other never does.
| Direction | Tested | Fail to round-trip |
|---|---|---|
| Whole inches → cm → inches | 1,000 | 0 |
| Whole cm → inches → cm | 1,000 | 611 |
The page shows 5 decimal places, which is far more than the terminating direction needs and not enough for the repeating one. The first centimetre to fail is 3 — 1 and 2 survive, so it is not that everything breaks. And note what is not the cause: the two units are only 2.54 apart in size, so unlike a converter spanning orders of magnitude this asymmetry is nothing to do with magnitude. It is entirely about which way round the decimal expansion terminates.
And 2.54 is not quite 2.54 inside the machine
2.54 is exact in decimal and not representable in binary — its denominator is 50, which is not a power of two — so the value the computer holds is very slightly off and every product inherits it. In practice it never shows. Sweeping 100,000 whole inches, the worst gap between the computed product and the exact decimal answer is 2.9e-11 cm, about 3.4 billion times finer than a millimetre. And 864 of the first 1,000 whole inches land exactly on a whole hundredth of a centimetre anyway, because the rounding happens to fall the right way — including every everyday size: 1, 2, 3, 5, 7, 10 and 12 inches are all exact.
This is the mirror image of a converter dividing by eight, where the factor is binary-exact and the display is the only place error can enter. Here the factor carries a residue and the display hides it completely. Everything downstream inherits the definition cleanly: a foot is exactly 30.48 cm, a yard 91.44, and a mile 160,934.4.
How to use
- Type a value into either the inches box or the centimetres box.
- The other box updates as you type — there is no convert button.
- For a height, enter the total inches: 5 feet 9 is 69.
- Use the reference table for common sizes you look up repeatedly.
Frequently asked questions
How many centimetres is 1 inch?
Exactly 2.54 centimetres. This is a definition rather than a measurement: since the international yard and pound agreement of 1959, the inch has been defined as 2.54 cm precisely, so the conversion carries no rounding error at all. Going the other way, one centimetre is about 0.3937 inches.
What is 5 feet 9 inches in centimetres?
175.26 cm, usually written as 175 cm. To convert any height, first turn it into total inches by multiplying the feet by 12 and adding the remainder — 5 feet 9 is (5 x 12) + 9 = 69 inches — then multiply by 2.54.
Is there a quick way to estimate this in my head?
Multiply by 2.5 and add a little. For 40 inches, 40 x 2.5 = 100, and the true answer is 101.6, so you are within about 2%. The error is always in the same direction — the estimate is low — which makes it easy to correct for.
Why do screen sizes use inches when everything else is metric?
Screen and wheel sizes are measured diagonally in inches by industry convention that predates metrication and never changed, much like pipe diameters and vinyl records. A 27-inch monitor is 68.6 cm corner to corner. Note that the figure is the diagonal, not the width, so a 27-inch 16:9 display is only about 59.8 cm wide.
Are US inches and UK inches the same?
They are now. Before 1959 the US and imperial inches differed very slightly, which mattered for precision engineering but not for anything else. The US retains a separate survey inch for legacy land records, larger by two parts per million, but it was officially retired for new work at the end of 2022.
How do I convert a fraction like 3/8 inch?
Divide the top by the bottom first, then multiply by 2.54. Three-eighths is 0.375 inches, which is 0.9525 cm — just under 10 mm. Fractional inches are still standard in hardware, so a 3/8 bolt and a 10 mm bolt are close enough to look interchangeable and are not.
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