Centimetres to inches

Inches are spoken in halves and quarters rather than decimals, and a tape measure is marked the same way, so the last step of this conversion is usually a second conversion. To read a decimal answer off a tape, multiply the fractional part by 16 and round to the nearest whole number: 0.7 in becomes 11.2, so eleven-sixteenths is the nearest mark. Sixteenths sit on 0.0625 steps, which is why 0.375 is exactly three-eighths and 0.8125 is exactly thirteen-sixteenths. Engineering drawings skip fractions altogether and work in thousandths, where one thou is 0.00254 cm.

Before 1959 there were two inches, and they disagreed several decimal places down. The United States defined its inch through the Mendenhall Order of 1893, which fixed the metre at exactly 39.37 inches and made the inch 2.54000508 cm. The British imperial inch came out a shade under 2.54. The gap was a few parts per million, which is nothing on a tape measure and a serious problem for gauge blocks, because a set certified against one standard would not certify against the other. The 1959 agreement replaced both with a round 2.54 and the difference disappeared.

Two everyday uses dominate this page. Height, where 180 cm is 70.87 in, or 5 feet 10.9 inches, and where the form asking for it almost always wants feet and inches rather than a decimal. And screens, where the quoted number is the diagonal and only the diagonal: a 27-inch monitor is 68.58 cm corner to corner and about 59.8 cm wide at a 16:9 shape, which is the measurement that decides whether it fits the shelf.