Fahrenheit to Celsius

The United States is the main country that uses Fahrenheit for weather, cooking and thermostats, joined by a few places in its orbit such as the Bahamas, the Cayman Islands and Liberia. Everyone else switched to Celsius during the metric changes of the 1960s and 1970s, although older people in the UK still describe a heatwave as being in the eighties or nineties.

So the conversion comes up whenever you read an American forecast, recipe or product manual. A US thermostat set to 72 °F holds the room at 22.2 °C; 65 °F is 18.3 °C. A forecast high of 100 °F is 37.8 °C, and 0 °F, a normal winter night in Minnesota, is −17.8 °C. A pool heated to 80 °F is 26.7 °C.

US recipes give oven heat in steps of 25 °F, which do not land on round Celsius numbers. 325 °F is 162.8 °C, 350 °F is 176.7 °C, 375 °F is 190.6 °C, 400 °F is 204.4 °C, 425 °F is 218.3 °C and 450 °F is 232.2 °C. Most cooks round to the nearest 5 or 10 on the dial, so 350 °F becomes 175 or 180 °C. If your oven is fan-assisted, many recipe writers also suggest lowering the Celsius figure by about 20 degrees.

Candy and frying thermometers need more care. The soft-ball stage at 235 °F is 112.8 °C and deep-frying oil at 375 °F is 190.6 °C; rounding by five degrees there changes the result.

The 98.6 °F figure is a converted round number. The German physician Carl Wunderlich published 37 °C as the average healthy temperature in 1868, and 37 × 1.8 + 32 happens to give 98.6 exactly, so the decimal suggests more precision than the original data had. Later studies put the average closer to 36.6–36.8 °C, or about 97.9–98.2 °F.

The mistake to avoid is multiplying before subtracting 32. 98.6 × 5/9 minus 32 gives 22.8, which is obviously not a body temperature. Subtract first, then scale. For the reverse direction, open Celsius to Fahrenheit.