How to convert Celsius to Fahrenheit and back

Multiply the Celsius temperature by 9/5 (1.8) and add 32. So 25°C is 25 × 1.8 + 32 = 77°F, and 100°C is 212°F.

The reason for the 32 is that the two scales start in different places: water freezes at 0°C and at 32°F.

Subtract 32 and multiply by 5/9. Normal body temperature of 98.6°F is (98.6 − 32) × 5/9 = 37°C.

Remember to subtract before you multiply. Doing it the other way round is the most common mistake.

Kelvin is Celsius plus 273.15, so 0°C is 273.15 K. Absolute zero, 0 K, is −273.15°C, the lowest temperature possible.

Handy reference points: water freezes at 0°C (32°F), a comfortable room is about 20°C (68°F), and water boils at 100°C (212°F) at sea level. The scales cross at −40.

To estimate Fahrenheit from Celsius, double it and add 30. For 20°C that gives 70°F, close to the exact 68°F. The error grows at extreme temperatures.

For exact answers, use the unit converter, which applies the formulas for you and rejects temperatures below absolute zero.