cm to Inches Converter

The exact figure

The number 2.54 is suspiciously round for a conversion between two independently developed systems, and that is because it was chosen rather than discovered. Until 1959 the inch was defined differently in different countries, all of them close to but not exactly 2.54 cm. The International Yard and Pound Agreement redefined the inch as precisely 2.54 cm, which had the elegant effect of making every imperial length unit an exact multiple of a metric one.

The United States kept a separate "survey inch" for land measurement, marginally larger, to avoid invalidating existing property records. It was formally retired at the end of 2022, so all US measurements now use the international inch.

The fraction problem

Metric measurements divide by ten; inches divide by two. A tape measure marked in sixteenths gives you 1/16, 1/8, 3/16 and so on, none of which correspond to round centimetre values. So a conversion that comes out as 11.811 inches is arithmetically right and practically useless at a workbench.

The nearest sixteenth to 11.811 is 11 and 13/16, which is 11.8125 — close enough that no saw will know the difference. This is why converting a metric plan to imperial measurements is genuinely awkward work rather than a lookup: every dimension needs rounding to a fraction your tools can actually cut, and those roundings accumulate across a design.

Height, and the conversion people actually want

Most searches for this conversion are about height, where the answer needs to be in feet and inches rather than inches alone. 180 cm is 70.87 inches, which is 5 feet 10.87 inches, which most people would round to 5 foot 11.

The arithmetic is: convert to inches, divide by twelve for the feet, and the remainder is the inches. 165 cm gives 64.96 inches, which is 5 feet 4.96 inches, or 5 foot 5 rounded. Passports, driving licences and medical records in the US record height in feet and inches while nearly every other country records centimetres, which is why the same person appears as two different numbers on two documents.

Screens, tyres and other diagonal measurements

A 27-inch monitor is not 27 inches wide. Display sizes are quoted as the diagonal, so a 27-inch 16:9 screen is about 59.8 cm wide and 33.6 cm tall. Buying a monitor to fit a specific shelf on the basis of its stated size is a common and expensive mistake.

Tyres mix the systems within a single code: a 225/45R17 tyre has a 225 mm width, a sidewall 45% of that height, and fits a 17-inch wheel. Bicycle frames, pipe fittings and camera sensors all have their own hybrid conventions. The conversion above handles the arithmetic; the convention is the part worth checking before you buy.

Frequently asked questions