Temperature Converter

Temperature is the one conversion on this site that cannot be done with a single multiplication, and understanding why explains most of the mistakes people make with it.

The scales have different zero points as well as different step sizes. Zero Celsius is water freezing; zero Fahrenheit is a brine mixture Fahrenheit could reproduce in his workshop; zero Kelvin is absolute zero. So converting means both scalingand shifting: multiply by 9/5, then add 32. Relationships like this are called affine rather than proportional.

That is why doubling a temperature does not double anything physical. 20°C is not twice as warm as 10°C — in Kelvin, where zero really is zero, they are 293 K and 283 K, four percent apart. Only on the Kelvin and Rankine scales does “twice as hot” mean anything.

The tool refuses temperatures below absolute zero — −273.15°C, −459.67°F, 0 K — because no such temperature exists. Converting one would produce an arithmetically valid, physically meaningless number.

An American recipe says to bake at 350°F and your oven is marked in Celsius.

Type 350, choose Fahrenheit: 176.67°C. In practice you set the oven to 175 or 180, whichever mark it has — and the reference row for “moderate oven” at 180°C confirms you are in the right region rather than out by a factor.

Now the one worth memorising. Type −40 in either scale and the other reads −40. It is the single point where the two scales cross, and it is a useful sanity check on any conversion: if your method does not give −40 for −40, your method is wrong.

A medical example. A thermometer reads 101.2°F and you are used to Celsius. That is 38.4°C — a real fever, just above the 38.5 reference row. The rough shortcut of “subtract 30 and halve” gives 35.6°C, which would tell you the opposite. Around body temperature that shortcut is badly wrong, which is exactly where people reach for it.

Try entering −300°C. The tool refuses: absolute zero is −273.15°C, and there is nothing below it. A converter that happily returned −508°F for that input would be doing arithmetic without meaning.