How to Convert Celsius to Fahrenheit (and Back)

To convert Celsius to Fahrenheit, multiply by 9/5 (which is 1.8) and add 32: F = C × 9/5 + 32. To convert Fahrenheit to Celsius, reverse it — subtract 32 first, then multiply by 5/9: C = (F − 32) × 5/9.

Kelvin is even simpler, because it uses the same degree size as Celsius but starts at absolute zero. Add 273.15 to a Celsius value to get Kelvin, or subtract it to go back.

Take 20°C, a mild room temperature. Multiply by 1.8 to get 36, then add 32: that's 68°F. Going the other way, 98.6°F (body temperature) minus 32 is 66.6, times 5/9 gives 37°C.

A useful quirk: −40 is the one point where the two scales meet — −40°C equals −40°F exactly. And the boiling point of water, 100°C, is 212°F, while freezing, 0°C, is 32°F.

For a rough Celsius-to-Fahrenheit estimate without a calculator, double the Celsius number and add 30. For 20°C that gives 70°F, close to the exact 68°F — fine for knowing how to dress, though off by a few degrees.

When you need the exact answer — for cooking, science, or a fever reading — use the full formula or the converter, which updates all three scales as you type and lists the common values people look up most.