How to Convert Temperature: Celsius, Fahrenheit & Kelvin
Updated: July 2026
Temperature is measured in three main scales: Celsius, Fahrenheit and Kelvin. Converting between them trips a lot of people up because, unlike most conversions, it is not a simple multiplication. This guide gives you the formulas and the shortcuts.
Celsius to Fahrenheit
The formula is: °F = (°C × 9/5) + 32. So 20°C = (20 × 1.8) + 32 = 68°F. A useful shortcut for a rough estimate: double the Celsius and add 30.
Fahrenheit to Celsius
The reverse: °C = (°F − 32) × 5/9. So 100°F = (100 − 32) × 0.556 = 37.8°C. Notice 37°C (98.6°F) is normal body temperature.
What about Kelvin?
Kelvin is the scientific scale. It uses the same size degree as Celsius but starts at absolute zero: K = °C + 273.15. So 0°C = 273.15 K. There are no negative Kelvin values because you cannot go below absolute zero.
Key reference points
- Water freezes: 0°C = 32°F = 273.15 K
- Water boils: 100°C = 212°F = 373.15 K
- Body temperature: 37°C = 98.6°F
- Room temperature: ~20°C = 68°F
Frequently asked questions
How do I convert Celsius to Fahrenheit?
Use °F = (°C × 9/5) + 32. For example, 25°C = (25 × 1.8) + 32 = 77°F. For a rough estimate, double the Celsius value and add 30.
What is the difference between Celsius and Kelvin?
Kelvin uses the same degree size as Celsius but starts at absolute zero. To convert, add 273.15 to the Celsius value. So 0°C equals 273.15 K.
At what temperature are Celsius and Fahrenheit equal?
They are equal at −40 degrees: −40°C = −40°F. It is the only point where both scales read the same number.