How to convert Fahrenheit to Celsius
Subtract 32 from the temperature in Fahrenheit and then multiply by 5/9 (or divide by 1.8). The subtraction moves the zero point, since water freezes at 32 °F and at 0 °C, and the multiplication shrinks the degree: a Fahrenheit degree is 5/9 of a Celsius degree.
Worked example: a US forecast says 72 °F. 72 − 32 = 40, and 40 × 5/9 = 22.22 °C. Normal body temperature, 98.6 °F: 98.6 − 32 = 66.6, and 66.6 × 5/9 = 37 °C. A cold morning at 5 °F: 5 − 32 = -27, and -27 × 5/9 = -15 °C.
Subtract before you multiply. Doing it the other way round gives the wrong answer. Check any result against a fixed point: 32 °F must be 0 °C and 212 °F must be 100 °C.
US weather and cooking in Celsius
- Weather: 0 °F is -17.8 °C; 20 °F is -6.7 °C; 32 °F is 0 °C; 50 °F is 10 °C; 60 °F is 15.6 °C; 70 °F is 21.1 °C; 80 °F is 26.7 °C; 90 °F is 32.2 °C; 100 °F is 37.8 °C.
- Ovens: 325 °F is 163 °C; 350 °F is 177 °C; 375 °F is 191 °C; 400 °F is 204 °C; 425 °F is 218 °C; 450 °F is 232 °C. Fan or convection ovens usually need about 20 °C less than the number in a conventional oven recipe, so check the recipe.
- Body: 98.6 °F is 37 °C. A fever of 100.4 °F is 38 °C, the figure at which many guidelines call a temperature a fever.
- Water: it freezes at 32 °F and boils at 212 °F at sea level, which are exactly 0 °C and 100 °C.
The subtract 30 and halve shortcut
Subtracting 30 and halving is close for mild temperatures. At 50 °F it gives 10 °C, which is exact, and at 68 °F it gives 19 °C where the exact answer is 20. It runs low in the heat: at 86 °F it gives 28 °C where the exact answer is 30.
For a closer estimate, subtract 32 and then halve, then add a tenth of that. For 86 °F: 54 halved is 27, plus 2.7 is 29.7, close to 30. Use the converter above when it matters.