Celsius to Fahrenheit
Convert °C to °F with the exact formula, a reference table of weather and body temperatures, and a mental shortcut that works.
Runs in your browser
The formula
°F = (°C × 9/5) + 32Temperature conversion needs both a ratio and an offset, because the two scales disagree about where zero is. Celsius puts 0° at the freezing point of water; Fahrenheit puts water’s freezing point at 32°. This is why you cannot convert a temperature by multiplying alone, and why a difference of 1 °C is 1.8 °F, while a reading of 1 °C is 33.8 °F.
Going the other way: °C = (°F − 32) × 5/9. See
fahrenheit to celsius.
Reference points
| Celsius | Fahrenheit | |
|---|---|---|
| −40 °C | −40 °F | the scales cross |
| −17.8 °C | 0 °F | |
| 0 °C | 32 °F | water freezes |
| 10 °C | 50 °F | cool day |
| 20 °C | 68 °F | room temperature |
| 25 °C | 77 °F | warm day |
| 30 °C | 86 °F | hot day |
| 37 °C | 98.6 °F | body temperature |
| 40 °C | 104 °F | |
| 100 °C | 212 °F | water boils at sea level |
The mental shortcut
Double it and add 30. For 20 °C that gives 70 °F against a true 68 °F, close enough to decide what to wear.
The approximation drifts because you are using a factor of 2 where the real one is 1.8. It is at its best in ordinary weather (roughly 5-25 °C) and increasingly wrong outside that range: at 100 °C it gives 230 °F against a true 212 °F.
Body temperature and the 98.6 myth
The familiar 98.6 °F is not a measured constant. It is 37 °C converted, and the original 19th-century figure was itself a rounded average. Modern studies put normal adult body temperature somewhat lower, around 36.6 °C (97.9 °F), and it varies by person, by site of measurement, and by time of day. The false precision of “98.6” is entirely an artefact of converting a rounded Celsius number.
Why −40 is the same in both
Setting °C = °F in the formula and solving gives exactly −40. It is the one temperature
where the two scales agree, which makes it a handy sanity check on any conversion code.