Convert between Celsius, Fahrenheit, Kelvin and Rankine, with an absolute zero check.
Enter the values to see the result here.
Formula
Temperature scales differ by an offset as well as a unit size, so each conversion goes through Celsius:
∘F=59∘C+32,K=∘C+273.15,∘R=59(∘C+273.15)
The reverse of the first is ∘C=95(∘F−32). Any value below −273.15∘C is rejected because nothing can be colder than absolute zero.
Worked example
Convert 72 °F to Celsius and Kelvin.
72−32=40, then 40×5/9=22.22∘C.
Kelvin: 22.22+273.15=295.37 K.
Body temperature in the other direction: 37×9/5+32=98.6∘F.
Common mistakes
Multiplying by 9/5 without adding 32, which is only valid for temperature differences, not readings.
Applying the 5/9 factor before subtracting 32.
Writing negative kelvins or a degree sign on kelvin.
Using the double-and-add-30 trick for cooking, where a 12 °F error ruins a recipe.
Frequently asked questions
Why does -40 appear the same in Celsius and Fahrenheit?
Because both scales are straight lines, they cross once. Setting C = F in F = 1.8C + 32 gives C = -40.
What is a quick mental conversion from Celsius to Fahrenheit?
Double it and add 30. 20 °C becomes 70 °F (exact is 68). Good enough for weather, not for baking or medicine.
What is the difference between Kelvin and Celsius?
A kelvin step is the same size as a Celsius degree, but Kelvin starts at absolute zero, so K = °C + 273.15. Kelvin has no degree sign and no negative values.
Why do ovens use different numbers abroad?
Gas marks, Fahrenheit and Celsius are all used. 180 °C is 356 °F, usually written as 350 °F; the difference is within normal oven tolerance.