Temperature Converter
Temperature Converter
Converting between Celsius, Fahrenheit and Kelvin
Celsius and Fahrenheit are both arbitrary scales: someone picked the zero point, and the two people who picked disagreed. That is why converting between them takes a multiplication and an addition, where converting metres to feet takes only a multiplication. Kelvin shares its degree size with Celsius but puts zero at absolute zero, the point where a substance has no thermal energy left to give up.
The multiplication is 9/5 because there are 180 Fahrenheit degrees between the freezing and boiling points of water where there are 100 Celsius degrees, and 180/100 reduces to 9/5. The addition is 32 because Fahrenheit put water's freezing point there rather than at zero.
Formulas
Worked examples
| From | To | Result |
|---|---|---|
| 100 °C | °F | 212 °F |
| 37 °C | °F | 98.6 °F |
| 0 °C | °F | 32 °F |
| 98.6 °F | °C | 37 °C |
| 72 °F | °C | 22.2222 °C |
| 350 °F | °C | 176.667 °C |
| 25 °C | K | 298.15 K |
Results are computed with the same factors the converter above uses, rounded to six significant figures.
Reference points
| °C | °F | K |
|---|---|---|
| −273.15 | −459.67 | 0 |
| −40 | −40 | 233.15 |
| 0 | 32 | 273.15 |
| 20 | 68 | 293.15 |
| 37 | 98.6 | 310.15 |
| 100 | 212 | 373.15 |
| 180 | 356 | 453.15 |
| 200 | 392 | 473.15 |
| 220 | 428 | 493.15 |
The last three are common oven settings. 180 °C is the temperature most European recipes mean by "moderate".
Worth knowing
- −40 is the one reading where the two scales agree: −40 °C is −40 °F. It falls out of solving °C = °C × 9/5 + 32.
- A temperature difference does not take the offset. A rise of 1 °C is a rise of 1.8 °F, not 33.8 °F. This is the single most common temperature conversion bug.
- Kelvin is written without a degree sign: 300 K, not 300 °K. It was dropped by international agreement in 1967.
Frequently asked questions
- What is the quickest way to estimate Celsius to Fahrenheit in your head?
- Double it and add 30. For 20 °C that gives 70 against a true 68, and for 30 °C it gives 90 against a true 86. The error grows as you move away from about 10 °C, but it is close enough for weather.
- Why do Celsius and Fahrenheit read the same at −40?
- Because the two scales converge. Setting the conversion equal to its own input and solving gives x = 9x/5 + 32, which resolves to x = −40. It is the only value where the different degree sizes and the 32-degree offset cancel out exactly.
- Do I convert a temperature difference the same way?
- No. Absolute readings take the offset, differences do not. To convert a difference from Celsius to Fahrenheit, multiply by 9/5 and stop. A 5 °C rise is a 9 °F rise. Applying the +32 to a difference is the reason so many climate and engineering figures get misquoted.
- Why does science use Kelvin?
- Because gas laws, thermodynamics and radiation physics all involve ratios of temperature, and ratios are meaningless on a scale with an arbitrary zero. Doubling 10 °C to 20 °C does not double anything physical, but doubling 300 K to 600 K genuinely doubles the average thermal energy.