About this calculator
Enter a temperature, choose its scale, and this converter shows it in Celsius, Fahrenheit and Kelvin at once, with the formula worked through.
It also warns you if the value is below absolute zero, the coldest temperature that can exist.
Worked examples
Real numbers, worked out by the same calculator. Press “Use these numbers” to try one above.
100 °C (boiling water)
- Fahrenheit
- 212 °F
- Celsius
- 100 °C
- Kelvin
- 373.15 K
100 °C is 212 °F and 373.15 K.
Show the working
- Fahrenheit = Celsius × 9 ÷ 5 + 32 = 100 × 9 ÷ 5 + 32 = 212 °F.
- Kelvin = Celsius + 273.15 = 373.15 K.
37 °C (body temperature)
- Fahrenheit
- 98.6 °F
- Celsius
- 37 °C
- Kelvin
- 310.15 K
37 °C is 98.6 °F and 310.15 K.
Show the working
- Fahrenheit = Celsius × 9 ÷ 5 + 32 = 37 × 9 ÷ 5 + 32 = 98.6 °F.
- Kelvin = Celsius + 273.15 = 310.15 K.
32 °F (freezing point of water)
- Celsius
- 0 °C
- Fahrenheit
- 32 °F
- Kelvin
- 273.15 K
32 °F is 0 °C and 273.15 K.
Show the working
- Celsius = (Fahrenheit − 32) × 5 ÷ 9 = (32 − 32) × 5 ÷ 9 = 0 °C.
- Kelvin = Celsius + 273.15 = 273.15 K.
−40 (the same in Celsius and Fahrenheit)
- Fahrenheit
- -40 °F
- Celsius
- -40 °C
- Kelvin
- 233.15 K
-40 °C is -40 °F and 233.15 K.
Show the working
- Fahrenheit = Celsius × 9 ÷ 5 + 32 = -40 × 9 ÷ 5 + 32 = -40 °F.
- Kelvin = Celsius + 273.15 = 233.15 K.
0 K (absolute zero)
- Celsius
- -273.15 °C
- Fahrenheit
- -459.67 °F
- Kelvin
- 0 K
0 K is -273.15 °C and -459.67 °F.
Show the working
- Celsius = Kelvin − 273.15 = 0 − 273.15 = -273.15 °C.
- Fahrenheit = Celsius × 9 ÷ 5 + 32 = -459.67 °F.
The formulas
- Celsius to Fahrenheit: °F = °C × 9 ÷ 5 + 32
- Fahrenheit to Celsius: °C = (°F − 32) × 5 ÷ 9
- Celsius to Kelvin: K = °C + 273.15
- Kelvin to Celsius: °C = K − 273.15
For example, 100 °C × 9 ÷ 5 = 180, plus 32 is 212 °F, and 100 + 273.15 is 373.15 K.
Useful reference points
| °C | °F | K | |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| A comfortable room | 20 | 68 | 293.15 |
| Normal body temperature | 37 | 98.6 | 310.15 |
| Water boils (at sea level) | 100 | 212 | 373.15 |
Why Kelvin?
The Kelvin scale starts at absolute zero, where particles have as little energy as is physically possible, so it has no negative numbers. Scientists use it because temperature can then be compared in ratios: 400 K is twice as hot as 200 K in a way that 40 °C is not twice as hot as 20 °C. A kelvin is the same size as a degree Celsius, which is why the conversion is just adding 273.15. Note that Kelvin is written without a degree symbol.
A curiosity: −40
−40 is the one temperature that's the same on the Celsius and Fahrenheit scales. Try it in the examples above.
Frequently asked questions
How do I convert Celsius to Fahrenheit?
Multiply by 9, divide by 5, then add 32. 20 °C is 20 × 9 ÷ 5 + 32 = 68 °F.
How do I convert Fahrenheit to Celsius?
Subtract 32, multiply by 5, then divide by 9. 98.6 °F is (98.6 − 32) × 5 ÷ 9 = 37 °C.
What is absolute zero?
The lowest temperature possible: 0 K, −273.15 °C or −459.67 °F. Nothing can be colder.
How do I convert Celsius to Kelvin?
Add 273.15. The size of one degree is the same in both scales.
Why is Kelvin written without a degree sign?
It's treated as a unit in its own right, like metres or kilograms, rather than a degree on a scale.
Formulas tested against hand-worked answers. Last reviewed 29 September 2026. These calculators do arithmetic only; they are not financial, tax or legal advice.