Centimeters to Inches

Until the middle of the twentieth century the inch was not the same everywhere: the British inch and the American inch differed in the sixth significant digit, which mattered for precision engineering and wartime parts. In 1959 the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed on the international yard of exactly 0.9144 m. A yard is 36 inches, which makes the inch exactly 2.54 cm, and the factor has not changed since.

Because 2.54 is exact, a result such as 25.4 mm = 1 in is not an approximation but a definition. The decimal tail appears only in the other direction: 1 cm = 0.393700787… in, a number that never ends. That is why it is easier to divide by 2.54 than to multiply by 0.3937 when you work it out by hand.

The United States uses inches for almost everything in daily life, from body height to lumber. The UK and Canada are officially metric but people still give their height in feet and inches, and builders in both countries often talk about four-by-eight sheets. Worldwide, screens, tyres, bicycle wheels and pipe fittings are sold in inches even where everything else is metric: a 27-inch monitor has a diagonal of about 68.6 cm.

Paper is the other common crossover. US Letter is 8.5 × 11 in (21.59 × 27.94 cm) and A4 is 21.0 × 29.7 cm (8.27 × 11.69 in), so a layout made for one will not fit the other without scaling. If you are preparing a photo or scan at a physical size, resize the image in centimeters or work out the pixels with the pixel converter.

The most common slip is multiplying by 2.54 when you should divide. A quick check: inches are bigger than centimeters, so the number of inches is always smaller. If 50 cm comes out as 127, you went the wrong way; that is the inches to centimeters direction.

Heights trip people up next. 175 cm is 68.9 in, which is 5 ft 8.9 in, not 5.689 ft and not 6 ft 9. Divide by 2.54 to get inches, then split off whole feet of 12 inches and keep the remainder. For other lengths and every other category, the unit converter has the full list.