Temperature Converter
Celsius, Fahrenheit and Kelvin — edit any one and the other two follow.
Temperature is the conversion people get wrong most often, because it is the one that does not work by multiplication. Every other unit on this site scales from zero; the Fahrenheit and Celsius scales start in different places and count in different sized steps, so the conversion needs both an offset and a factor. Change any of the three boxes below and the other two follow.
| °C | °F | K |
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How it works
Type in any box
Celsius, Fahrenheit or Kelvin — the other two update instantly.
Check the formula
The exact relationships are shown under the inputs.
Use the reference points
Freezing, body temperature, boiling and absolute zero in all three scales.
Why temperature needs an offset and every other unit does not
Converting metres to feet is multiplication because both scales agree on where zero is: no length is no length. Celsius and Fahrenheit do not agree. Zero Celsius is the freezing point of water, zero Fahrenheit is roughly the freezing point of a brine solution, and the two are 32 degrees apart. On top of that, a Celsius degree is 1.8 times the size of a Fahrenheit degree.
So the conversion carries both corrections: multiply by 9/5 for the step size, then add 32 for the offset. Reversing it means subtracting first and then dividing — doing it in the other order is the single most common mistake, and it gives an answer that is wrong by about 18 degrees.
The offset also means differences convert differently from values. A rise of 10 °C is a rise of 18 °F, not 50 °F, because the offset cancels when you subtract two temperatures. Weather reports and equipment specifications routinely mix the two, so it is worth checking whether a figure is a temperature or a change in temperature before converting it.
Kelvin, absolute zero and the one crossing point
Kelvin removes the offset problem by starting at absolute zero, the point where a substance has given up all the thermal energy it can. That is −273.15 °C, and nothing can go below it — which is why Kelvin has no negative values and why the unit is written without a degree sign. A kelvin is the same size as a Celsius degree, so the conversion is a plain addition.
Since 2019 the kelvin has been defined by fixing the Boltzmann constant rather than by the properties of water, which makes it independent of any physical sample. In practice nothing changed at everyday temperatures; the redefinition matters at the extremes, where reproducing a water-based standard was the limiting source of error.
One arithmetic curiosity is worth remembering because it doubles as a sanity check: −40 is the same temperature on both the Celsius and Fahrenheit scales. It is the single point where the two lines cross. If a conversion you have done by hand does not agree with the tool, checking it against −40 usually reveals which way round the offset went.