Celsius and Fahrenheit converter

An American tells you it is 100 degrees outside, and you have no idea whether to take a coat. In the United States temperature is measured in degrees Fahrenheit, a scale that is both shifted and stretched: water freezes at 32 °F and boils at 212 °F. So between freezing and boiling there are 180 Fahrenheit degrees, against our 100, and one degree Celsius equals 1.8 degrees Fahrenheit.

Physicists also measure in kelvin. A kelvin is the same size as a degree Celsius, but it starts at absolute zero: K = C + 273.15. Nothing colder than 0 K, that is −273.15 °C, exists, so the calculators will not work out anything below that.

From degrees Celsius to Fahrenheit

From degrees Fahrenheit to Celsius

How it works

Converting between the scales uses these formulas:

$$F=C\cdot 1.8+32,C=\frac{F-32}{1.8}$$

For a room temperature of 20 °C, multiply by 1.8 and add 32:

$$F=20\cdot 1.8+32=36+32=68\text{°F}$$

In kelvin it is 20 + 273.15 = 293.15 K.

Now the other way round, with that American 100 °F. First subtract 32, and only then divide:

$$C=\frac{100-32}{1.8}=\frac{68}{1.8}\approx 37.78\text{°C}$$

So you can leave the coat at home. A common mistake is to do it in the wrong order, dividing first and then subtracting. You would get $100÷1.8-32\approx 23.6$ °C, which is a pleasant spring day rather than a heatwave.

To estimate in your head, subtract 30 from the Fahrenheit temperature and halve the result: $(100-30)÷2=35$ °C. Around room temperatures you will be out by a degree or two.