Temperature units explained
Temperature is the one category where units do not simply scale — the scales start in different places, so converting means shifting as well as multiplying. Two of them begin at absolute zero and two begin somewhere a person chose.
Degree Celsius °C
1 °C = 1 °C Exact
Zero at the freezing point of water, a hundred at its boiling point, which makes it easy to reason about weather and cooking. Anders Celsius originally had it upside down — zero for boiling, a hundred for freezing — and it was reversed after his death.
Degree Fahrenheit °F
1 °F = -17.222222 °C Exact
Zero was set at the temperature of a brine mixture and 96 at roughly human body heat, which is why the numbers look arbitrary: they are. The practical argument in its favour is resolution for weather, since a degree Fahrenheit is a smaller step, and the zero-to-hundred range covers most of what people actually experience outdoors.
Kelvin K
1 K = -272.15 °C Exact
The SI unit, starting at absolute zero, where molecular motion stops. A kelvin is the same size as a degree Celsius, so the conversion is just an offset of 273.15. Written without a degree sign and without the word degree — 300 K, read as three hundred kelvin.
Degree Rankine °R
1 °R = -272.594444 °C Exact
Fahrenheit's absolute scale: the same size step, but starting at absolute zero instead of a brine solution. Largely confined to American thermodynamics and aerospace work, where it lets engineers keep Fahrenheit-sized degrees in equations that require an absolute temperature.