Quick and easy Fahrenheit to Celsius conversion
There's a simple rule to convert Fahrenheit to Celsius that should be good enough for general use. Simply take 30 off the Fahrenheit value, and then half that number.
Note that this value isn’t perfect, but it might save you having to reach for a calculator (or our site!)
Definition of Fahrenheit and Celsius
In the Fahrenheit scale, water freezes at 32 degrees, and boils at 212 degrees. Boiling and freezing point are therefore 180 degrees apart. Normal body temperature is considered to be 98.6 °F (in real-life it fluctuates around this value). Absolute zero is defined as -459.67°F.
The Celsius scale is nowadays set in such a way that Zero degrees C is the temperature at which ice melts (note : not the temperature at which it freezes, which is different!) . At the other end of the scale, 100 degrees Celsius is the boiling point of water.
The scientific definition of Celsius is now defined against degrees Kelvin. Zero degrees Celsius is 273.15K. One degree Celsius is equal to one Kelvin, so we can say that the boiling point of water is equal to 273.15 + 100 = 373.15 Kelvin.
Fahrenheit to Celsius formula
Why is converting Fahrenheit to Celsius so complicated?
Most things we measure – length, width, time etc. have one thing in common – their values all start from zero. We all know exactly how long zero centimeters or inches is, and can convert zero of any of those units into another type of unit very easily. Zero centimeters = zero meters = zero inches. Taking inches and centimeters as an example, to go from zero inches to 1 inch we need to add one inch. So far, so obvious.
Similarly, to go from zero centimeters to 1 centimeter, we need only add 1 centimeter. The only difference between adding one inch or one centimetre is the amount of distance we're adding. The relationship between an inch and a centimetre is that 1 inch is 2.54 centimetres. So we can say that adding 1 inch is the same as adding 2.54 centimetres. Because they both start at zero, the formula to convert between the two very easy (in = cm * 0.39370)
Temperature units aren't built in the same simple way, because they don't all start in the same place at zero. If we pegged absolute zero to be 0°F, 0°C and 0K, converting between them would be much easier, but Fahrenheit and Celsius were defined before we could tell where absolute zero was, and as a result Fahrenheit, Celsius and Kelvin all start from different values.
Because these temperature units don’t share a common zero point, we need to add or subtract an offset before we do our division or multiplication. It's not a difficult extra step, but it seems to be something that can cause confusion. As a rough rule of thumb:
To go from Fahrenheit to Celsius, take 30 off the Fahrenheit value, and then half that number.
For a 100% accurate answer, subtract 32 and divide by 1.8 (or use the calculator above!)