CM to inches

Type a value in centimetres and read the inches. The number you get is not an approximation of a measured relationship: since 1959 the inch has been defined as exactly 2.54 centimetres, so the division is exact and only the display is rounded. Below the converter there is a complete table from 1 to 100 cm that you can copy in one go, and an explanation of the two things that actually trip people up — rounding, and inch fractions.

The conversion, and why it is exact

Inches = centimetres ÷ 2.54. Going the other way, centimetres = inches × 2.54. There is no third decimal hiding in that constant and no regional variation to worry about, because 2.54 is a definition rather than a measurement.

That definition came from the International Yard and Pound Agreement of 1959, signed by the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa. Before it, the inch was defined separately in each country and the values disagreed slightly: the US inch and the imperial inch differed by roughly two parts per million. Irrelevant for a bookshelf, but not for surveying or precision machining, which is exactly why the agreement happened.

One legacy of that history survives in the United States: the survey foot, used in land records, was kept on the old definition until the end of 2022. If you are reading old cadastral documents the difference is about 3 mm per kilometre. For everything else, 2.54 is the whole story.

Doing it in your head, and how close you get

Divide by 2.5 and then take off about one and a half per cent. Dividing by 2.5 is easy — double the number and move the decimal point one place left — and it overstates the answer by 1.57 per cent, which is close enough to judge whether a parcel fits in a box.

Two anchors are worth memorising because they cover most everyday cases. 2.5 cm is almost exactly one inch (0.984 in), and 30 cm is almost exactly one foot (11.81 in). Both are within two per cent, and both are the reason a "30 cm ruler" and a "12 inch ruler" look interchangeable but are not.

The reverse direction is easier: inches × 2.5 plus a bit under two per cent. For a 15 inch laptop that gives 37.5 cm against a true 38.1 cm.

Inch fractions, which is where most conversions actually fail

A tape measure is not marked in decimals. It is marked in halves, quarters, eighths and sixteenths of an inch, so a converter that tells you 3.937 inches has answered a question you cannot act on directly.

The arithmetic is straightforward once you know it. Multiply the fractional part by 16 and round to a whole number: 0.937 × 16 = 14.99, so 3.937 in is 3 and 15/16 inches. For thirty-seconds, multiply by 32 instead. Working the other way, a marking of 5/8 is 0.625 in, which is 1.5875 cm.

This tool gives you decimals, not fractions — we would rather say that than have you assume otherwise. If you are cutting timber or drilling, convert to sixteenths with the multiplication above, and remember that the saw blade itself is about 3 mm wide, which is bigger than the rounding error you were worrying about.

Where this conversion actually comes up

Screens are the most common case, and they are also the one where people get it wrong. A television or monitor size is the diagonal, in inches, of the panel only — not the bezel and not the stand. A 55 inch television is 139.7 cm corner to corner, and roughly 121 cm wide, which is the number that decides whether it fits your cabinet.

Clothing sizes are the second case. Waist and chest measurements on US and UK labels are in inches, so a 32 inch waist is 81.3 cm. Shoe sizing does not work this way at all — those scales are not simple unit conversions and a converter cannot help you.

Then there is everything mechanical: bicycle wheels and frames, plumbing, screw and bolt sizes, camera filter threads, luggage limits set by airlines in inches, and paper sizes outside the A series. Letter paper is 8.5 × 11 inches, which is 21.59 × 27.94 cm — very close to A4 at 21 × 29.7 cm, which is why documents laid out for one and printed on the other lose their bottom margin.

Why other converters sometimes show 0.9999999999

Computers store decimal fractions in binary, and 2.54 has no exact binary representation. Convert 1 cm to inches and back and the raw arithmetic can land on 0.9999999999999999 instead of 1. That is not a bug in the conversion; it is what floating point does.

The fix is to round for display to a fixed number of significant figures rather than a fixed number of decimals. This page uses six significant figures by default, which is enough that converting a value out and back returns exactly what you started with. If you switch the setting to two decimal places you get a friendlier number, but round-tripping stops working — 1 cm becomes 0.39 in, and 0.39 in comes back as 0.99 cm. The page says so at the time rather than letting you discover it.

We also do not insert thousands separators into the result. A number you are about to paste into a spreadsheet or a piece of code should not contain commas.