How to convert Celsius to Fahrenheit
Multiply the temperature in Celsius by 9/5, which is 1.8, and then add 32. The multiplication changes the size of the degree (a Celsius degree is 1.8 Fahrenheit degrees) and the 32 moves the zero point (water freezes at 0 °C and at 32 °F).
Worked example: a forecast says 25 °C. 25 × 1.8 = 45, and 45 + 32 = 77 °F. A fever of 38.5 °C: 38.5 × 1.8 = 69.3, plus 32 is 101.3 °F. A cold day at -10 °C: -10 × 1.8 = -18, plus 32 is 14 °F.
Add the 32 after you multiply. If you add it first and then multiply, you get the wrong answer. Check any result against a fixed point: 0 °C must be 32 °F and 100 °C must be 212 °F.
Weather, body and oven temperatures
Rows below come straight from the formula:
- Weather: -20 °C is -4 °F; -10 °C is 14 °F; 0 °C is 32 °F; 10 °C is 50 °F; 15 °C is 59 °F; 20 °C is 68 °F; 25 °C is 77 °F; 30 °C is 86 °F; 35 °C is 95 °F; 40 °C is 104 °F.
- Body temperature: 36.5 °C is 97.7 °F, 37 °C is 98.6 °F, and 38 °C is 100.4 °F. Many health services count 38 °C or above as a fever.
- Ovens: 160 °C is 320 °F; 180 °C is 356 °F; 190 °C is 374 °F; 200 °C is 392 °F; 220 °C is 428 °F; 230 °C is 446 °F. A recipe written for 180 °C is commonly given as 350 °F in the US, a rounding of 356.
- Water: it freezes at 0 °C (32 °F) and boils at 100 °C (212 °F) at sea level. Water heaters are often set to about 50 or 60 °C, which is 122 or 140 °F.
The double it and add 30 shortcut
Doubling and adding 30 is easy to do in your head and gets close when the temperature is mild. It uses 2 instead of 1.8 and 30 instead of 32. At 10 °C it gives 50 °F, which is exactly right. It overshoots as it gets warmer: at 20 °C it gives 70 °F where the exact answer is 68, and at 30 °C it gives 90 °F where the exact answer is 86.
A closer way to do the same sum in your head: double it, take off 10 percent of that, then add 32. For 30 °C: 60 less 6 is 54, plus 32 is 86. This is the exact formula in disguise, because 1.8 is 2 minus 0.2.