Convert inches to centimeters

One inch equals exactly 2.54 centimeters. The value is exact by definition rather than measurement — the international yard and pound agreement of 1959 fixed it, and the National Institute of Standards and Technology lists it as an exact conversion in Special Publication 811. Multiply inches by 2.54 to get centimeters; divide by 2.54 to go back.

Before 1959, the United States and the United Kingdom used slightly different inches, apart by about two parts per million. Irrelevant on a tape measure, the gap mattered in surveying and precision manufacturing. Six countries signed an agreement that year setting a single value for both the inch and the pound, and 2.54 cm has been exact since.

The inch is much older than that. A 1324 English statute defined it as three barleycorns, dry and round, laid end to end. The standard survives in an unexpected place: shoe sizes still step by one third of an inch, one barleycorn per size.

Screen size is the diagonal of the visible display, so a 27-inch monitor measures 68.58 cm corner to corner, not across the top. The convention comes from cathode-ray televisions: picture tubes were round glass measured by outside diameter, while only a rectangular portion actually showed an image. The bigger number made better marketing, and it survived into LCD and OLED.

Clothing sizes. A W32 jean has a 32-inch waist, or 81.28 cm, but German and European menswear labels that as a 46 or 48. The relationship is a sizing chart shaped by cut and vanity sizing, not a fixed ratio. Converting the number gives you a measurement, not a size. For workshop and technical work the same inch converts to millimetres — see inches to mm.

Six national laboratories, by agreement. The value was adopted by the US National Bureau of Standards (now NIST), in an announcement effective 1 July 1959, which fixed the yard at 0.9144 m and with it the inch at exactly 25.4 mm. NIST Special Publication 811, the US guide to the SI lists the inch as 2.54 × 10⁻² m with no rounding, and the United Kingdom wrote the same yard into its Weights and Measures Act 1963.