Temperature converter
Celsius, Fahrenheit and Kelvin together, with the sum shown underneath.
Two scales that disagree about zero
Most conversions are a single multiplication, because both units count from the same place and only the step size differs. Temperature is not like that. Celsius puts zero at the freezing point of water; Fahrenheit puts it about 18 degrees lower, at the coldest temperature its inventor could reliably produce with brine.
So converting takes two operations in the right order: multiply by 9/5, then add 32. Reversing it means subtracting 32 first, then multiplying by 5/9. Doing the offset and the scaling in the wrong order is the classic error, and it produces an answer that looks plausible.
Minus forty, and why it is worth remembering
The two scales cross at exactly one point, and it is -40. Minus forty Celsius is minus forty Fahrenheit. Two straight lines with different gradients meet exactly once, and that is where these meet.
It makes a good check on any conversion, including this one. If a converter does not give -40 for -40, it is wrong somewhere.
Doubling and adding thirty
For reading a foreign weather forecast, double the Celsius and add 30. It gives 20 °C as 70 °F against a true 68, and 25 °C as 80 against a true 77. Close enough to know whether to take a coat.
It drifts at the extremes, because the real factor is 1.8 rather than 2, so the error grows by 0.2 degrees for every degree. At oven temperatures it is out by forty or more, which is why the table above exists.
Oven temperatures and gas marks
British recipes still use gas marks, and they are not a conversion of anything. The scale is its own thing, roughly 25 °F per mark, starting at gas mark 1 for 140 °C.
Gas mark 4 is 180 °C or 350 °F, which is the default for most baking. Gas mark 6 is 200 °C or 400 °F. Gas mark 9 is 240 °C or 475 °F. A fan oven runs about 20 °C hotter than its dial suggests relative to a conventional one, so recipes usually say to drop the temperature by that much.
Kelvin, and the bottom of the scale
Kelvin measures from absolute zero, the point at which there is no thermal energy left to remove. That is -273.15 °C. A kelvin is the same size as a degree Celsius, so converting between them is only ever an addition or a subtraction of 273.15.
Because its zero is real rather than chosen, kelvin takes no degree sign and no “degrees”: it is 300 K, said as three hundred kelvin. And nothing can be below zero on it, which is why this page refuses to convert a temperature under absolute zero rather than returning a negative figure that means nothing.
Common questions
- How do I convert Celsius to Fahrenheit?
- Multiply by 9/5 and add 32. So 20 °C is 20 times 1.8, which is 36, plus 32, giving 68 °F. Going the other way, subtract 32 first and then multiply by 5/9. The order matters: the two scales have different zero points as well as different sized degrees, so the offset has to be dealt with separately from the scaling.
- Is there a quick way to do it in my head?
- Double it and add 30. That gives 20 °C as 70 °F against a true 68, and 30 °C as 90 against a true 86. It is within a few degrees across normal weather, which is all anybody needs when reading a foreign forecast. The error grows at the extremes, so do not use it for an oven.
- Why is -40 the same in both scales?
- Because the two lines cross there. Celsius and Fahrenheit run at different gradients from different starting points, so there is exactly one temperature where they agree, and it happens to be -40. It is a genuinely useful check: any conversion that does not give -40 °C = -40 °F is wrong.
- Why does Kelvin have no degree sign?
- Because it is an absolute scale rather than a relative one. Its zero is the actual bottom of the temperature range, not an arbitrary reference like the freezing point of water, so its steps are units rather than degrees. It is written 300 K, and said "300 kelvin", never "degrees kelvin".
- What is absolute zero?
- The coldest anything can be: -273.15 °C, -459.67 °F, or 0 K. It is where a substance has given up all the energy it can, and it cannot be reached, only approached. This page refuses to convert anything below it rather than returning a negative kelvin figure, which would be arithmetic pretending to be physics.