Temperature Converter

Enter a temperature in Celsius, Fahrenheit, Kelvin, Rankine or Réaumur and see it in all the others. Everything routes through Kelvin, which handles the part that trips up a naive conversion: these scales have different zero points as well as different degree sizes, so converting is an affine transformation rather than a multiplication.

That distinction has a practical consequence people get wrong constantly. A temperature and a temperature difference do not convert the same way. Twenty degrees Celsius is 68 Fahrenheit, but a rise of twenty degrees Celsius is a rise of thirty-six Fahrenheit, not sixty-eight — the offset applies to points on the scale and not to intervals between them. A thermal coefficient quoted per degree Celsius converts to Fahrenheit by dividing by 1.8 and nothing else. The same trap catches °C to K in the other direction: as an interval they are identical, since both use the same size degree, so a 5 °C rise is exactly a 5 K rise.

The scales themselves carry some history. Kelvin and Rankine both start at absolute zero and differ only in degree size — Kelvin uses Celsius-sized degrees, Rankine uses Fahrenheit-sized ones, which is why Rankine survives in American thermodynamics. Réaumur put water's freezing and boiling points at 0 and 80 and was the standard across much of Europe for a century; it persists mainly in some cheese and syrup recipes. Celsius and Fahrenheit cross at exactly −40, the one temperature where the two numbers agree.

Values below absolute zero are accepted as typed rather than rejected, because the tool is a converter and not a physics referee. Worth knowing that such a temperature is not merely impossible but backwards: in the systems where physicists do write a negative absolute temperature — a population inversion in a laser, for instance — that state is hotter than any positive temperature, not colder, because the sign flips at infinity rather than at zero.