Temperature Converter
Convert between Celsius, Fahrenheit, and Kelvin
Quick Reference
How to use this tool
- Enter a temperature value
- Select the unit you're converting from
- Select the unit you're converting to
- The result updates automatically
About this tool
Convert between the three temperature scales in use, with the result updating as you type and a colour cue for where the value sits. Celsius is the everyday scale almost everywhere, built around water freezing at 0 and boiling at 100 at sea level. Fahrenheit remains the everyday scale in the United States, where 0 and 100 sit near the extremes of a temperate climate rather than at any property of water. Kelvin is the scientific scale, starting at absolute zero, the point at which thermal motion stops. Temperature conversion is arithmetically different from converting length or weight, and this catches people out. Metres and feet share a zero, so converting is a single multiplication. Celsius and Fahrenheit do not: their zeros are 32 degrees apart and their degrees are different sizes. That is why converting Celsius to Fahrenheit means multiplying by nine fifths and then adding 32, and why doing those two steps in the wrong order gives a plausible-looking but wrong answer. Kelvin is the easy one, because it uses the same degree size as Celsius and only shifts the zero: add 273.15 to go from Celsius to Kelvin, subtract it to come back. It has a true zero, which is why it is the scale used whenever ratios have to mean something. 200 K really is twice 100 K; 20 degrees Celsius is not twice 10. Every conversion runs in your browser and nothing you enter is sent anywhere.
Frequently asked questions
How do I convert Celsius to Fahrenheit?
Multiply by nine fifths and add 32, so 20 degrees Celsius is 68 Fahrenheit. Going the other way, subtract 32 first and then multiply by five ninths. The order matters: subtracting after dividing gives the wrong answer.
Is there a quick way to estimate the conversion in my head?
Double the Celsius figure and add 30. It is a few degrees high in the middle of the range but close enough for weather: 20 gives 70 against a true 68, and 30 gives 90 against a true 86. Use the exact formula for anything that matters.
At what temperature are Celsius and Fahrenheit the same?
Minus 40. It is the single point where the two scales cross, which is why minus 40 needs no unit in practice.
What is Kelvin and when is it used?
The SI temperature scale, which starts at absolute zero, the point where thermal motion stops. It uses the same degree size as Celsius, so converting is just adding or subtracting 273.15. Science uses it because ratios only make sense on a scale with a true zero: 200 K really is twice as hot as 100 K, whereas 20 degrees Celsius is not twice 10.
Why does the world use two different temperature scales?
History. Fahrenheit came first and was adopted across the British Empire; Celsius, built around water freezing at 0 and boiling at 100, was cleaner and spread with metrication. The United States kept Fahrenheit for everyday use, which is why weather forecasts differ by country while science uses Celsius and Kelvin everywhere.
Does the converter handle negative temperatures?
Yes, for Celsius and Fahrenheit. Kelvin cannot go below zero, since that is absolute zero, so a negative Kelvin value is not a real temperature.