Celsius vs Fahrenheit: How to Convert Temperatures in Your Head
Temperature is the one common conversion you cannot do with a single multiplication. That is because the Celsius and Fahrenheit scales not only have different degree sizes, they also start from different zero points. Once you know the formula and a few anchor temperatures, you can estimate conversions in seconds.
Two scales, two logics
The Celsius scale, introduced by Anders Celsius in the 1740s, is anchored to water: 0 °C is where water freezes and 100 °C is where it boils at sea level. Fahrenheit, developed by Daniel Gabriel Fahrenheit in 1724, sets those same points at 32 °F and 212 °F. That means there are 180 Fahrenheit degrees between freezing and boiling and only 100 Celsius degrees, so each Celsius degree is 1.8 times larger than a Fahrenheit degree.
The exact formulas
- Celsius to Fahrenheit: multiply by 9/5 (or 1.8) and add 32.
- Fahrenheit to Celsius: subtract 32, then multiply by 5/9.
- Celsius to Kelvin: add 273.15.
For example, 25 °C × 1.8 = 45, plus 32 equals 77 °F. And 77 °F minus 32 is 45, times 5/9 is 25 °C. Try your own numbers in the Celsius to Fahrenheit converter, or go to absolute scale with Celsius to Kelvin.
Mental tricks that actually work
Celsius to Fahrenheit: double the number and add 30. For 20 °C: 20 × 2 = 40, plus 30 is 70 °F. The exact answer is 68 °F, so you are within two degrees. The shortcut gets less accurate at extremes, but for everyday weather it is excellent.
Fahrenheit to Celsius: subtract 30 and halve. For 80 °F: 80 − 30 = 50, half is 25 °C. The exact answer is 26.7 °C.
If you want a better estimate, remember that every 10 °C is 18 °F. Starting from 0 °C = 32 °F you get: 10 °C = 50 °F, 20 °C = 68 °F, 30 °C = 86 °F and 40 °C = 104 °F.
Temperatures worth knowing
| What | °C | °F |
|---|---|---|
| Water freezes | 0 | 32 |
| Cool room | 18 | 64 |
| Comfortable room | 21 | 70 |
| Normal body temperature | 37 | 98.6 |
| Fever threshold | 38 | 100.4 |
| Hot summer day | 35 | 95 |
| Water boils (sea level) | 100 | 212 |
| Typical baking oven | 180 | 356 |
The magic number: −40
There is exactly one temperature where both scales agree: −40. At −40 °C the thermometer also reads −40 °F, which makes it a handy check on your arithmetic.
Who uses which?
Almost every country uses Celsius for weather, cooking and medicine. Fahrenheit is used in the United States and a few territories, while scientists use Celsius and Kelvin everywhere. If you travel or cook across borders, knowing the formula and a few anchor points saves a lot of confusion.