Temperature Converter
Celsius, Fahrenheit, Kelvin and Rankine, all four at once, with the working shown so you can check it. Nothing is uploaded.
| °C | °F | K | |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| Same on both scales | −40 | −40 | 233.15 |
| Room temperature | 25 | 77 | 298.15 |
| Body temperature | 37 | 98.6 | 310.15 |
| Fever begins | 38 | 100.4 | 311.15 |
| Water boils | 100 | 212 | 373.15 |
| Baking oven | 180 | 356 | 453.15 |
How to use it
- Type the reading you have.
- Say which scale it is in.
- All four scales update together, and the line under the big number shows the arithmetic.
The two formulas worth memorising
Celsius to Fahrenheit: multiply by 9, divide by 5, add 32. Fahrenheit to Celsius: subtract 32, multiply by 5, divide by 9. The subtraction comes first going that way, and getting the order wrong is the single most common mistake.
For a rough figure in your head, double the Celsius and add 30. Thirty degrees becomes about 90, and the real answer is 86. It is close enough to know whether to take a jacket, and wrong enough that you should not cook with it.
Why minus forty is special
Fahrenheit rises 1.8 degrees for every 1 degree Celsius, and the two scales start from different places. Two straight lines with different slopes cross exactly once, and for these two it happens at −40. That is the one reading you never have to convert.
Kelvin, and the floor under everything
Kelvin measures from absolute zero, the point where there is no thermal energy left to remove. Nothing can be colder, which is why a negative kelvin figure is not a cold temperature but a mistake. Enter one here and the page says so instead of converting it.
A kelvin is the same size as a degree Celsius, so the two differ only by the 273.15 between their starting points. Write 300 K, not 300 degrees K, because a kelvin is a unit rather than a point on a marked scale. Rankine does for Fahrenheit what Kelvin does for Celsius, and turns up mainly in older engineering work.
Frequently asked questions
What is the formula for Celsius to Fahrenheit?
Multiply by nine, divide by five, then add thirty-two. So 37 degrees Celsius is 37 times 1.8, which is 66.6, plus 32, giving 98.6 degrees Fahrenheit. Going the other way, subtract 32 first and then multiply by five over nine.
At what temperature are Celsius and Fahrenheit the same?
At minus forty. Minus forty degrees Celsius is exactly minus forty degrees Fahrenheit, and it is the only point where the two scales meet, because one rises 1.8 degrees for every 1 degree of the other.
Why is Kelvin written without a degree sign?
Because a kelvin is a unit in its own right rather than a point on a scale marked off from something else. Water freezes at 273.15 K, and 0 K is absolute zero, the coldest anything can be. You write 300 K, never 300 degrees K.
Is anything I type sent anywhere?
No. The arithmetic happens in JavaScript inside your own browser. Nothing is uploaded and nothing is saved - close the tab and it is gone.