How to Convert Celsius to Fahrenheit (and Back): Formula and Quick Method
Temperature conversion is one of the most common calculations when travelling or reading international recipes. Here are two methods — exact and quick.
The Exact Formula
Celsius to Fahrenheit:
°F = (°C × 9/5) + 32
Fahrenheit to Celsius:
°C = (°F - 32) × 5/9
Worked Examples
20°C to Fahrenheit:
(20 × 9/5) + 32 = (20 × 1.8) + 32 = 36 + 32 = 68°F
98.6°F to Celsius:
(98.6 - 32) × 5/9 = 66.6 × 0.5556 = 37°C
The Quick Mental Method
For a fast estimate:
- Double the Celsius temperature
- Subtract 10%
- Add 32
Example: 25°C
- 25 × 2 = 50
- 50 - 5 = 45
- 45 + 32 = 77°F (exact: 77°F ✓)
The quick method is accurate within 1–2°F for typical temperatures.
Key Reference Temperatures
| °C | °F | What It Means | |----|-----|--------------| | -40 | -40 | Same in both scales | | 0 | 32 | Water freezes | | 20 | 68 | Room temperature | | 37 | 98.6 | Body temperature | | 100 | 212 | Water boils |
Celsius, Fahrenheit, and Kelvin
Kelvin is the SI unit for temperature, used in science:
K = °C + 273.15
Kelvin has no negative values — 0 K is absolute zero (-273.15°C), the coldest possible temperature.
| °C | °F | K | |----|-----|-----| | 0 | 32 | 273.15 | | 100 | 212 | 373.15 | | -273.15 | -459.67 | 0 |
Why Does the US Still Use Fahrenheit?
Most countries use Celsius (metric), but the US, Belize, Cayman Islands, and Palau use Fahrenheit. The US attempted to switch to metric in the 1970s but the effort was largely abandoned.
Fahrenheit was defined so that 0°F was the coldest temperature its inventor could achieve with salt water and ice, and 96°F was approximately body temperature (later recalibrated to 98.6°F).
Use our Temperature Converter to convert between Celsius, Fahrenheit, and Kelvin instantly.