Temperature Converter

Minus forty, the one place the two scales agree

−40 °C is −40 °F. It is the only temperature where the two scales give the same number, and it is not a coincidence so much as an inevitability: any two straight lines with different slopes cross exactly once. Set the conversion equal to itself and solve — C = 1.8C + 32 gives C = −40 — and that crossing point is where you land.

It is a genuinely useful anchor. Weather reports from Canada and Siberia quote it without a unit for exactly this reason, and if you remember one fixed point other than freezing, this is the one that tells you instantly which side of the conversion you are on.

The shortcut, and exactly how wrong it is

Double it and add 30. That is the conversion everyone uses, and it is worth knowing precisely how far it drifts, because the answer is tidy. The real formula is 1.8C + 32, so the shortcut overshoots by 0.2C − 2 degrees Fahrenheit. It is exact at 10 °C, and it moves 1 °F further out for every 5 °C you travel from there. At 0 °C it is 2 °F too low; at 30 °C it is 4 °F too high; at 100 °C it is 18 °F too high and useless. Inside the band that human weather actually occupies — roughly −10 to 35 °C — it is never more than 5 °F out, which is close enough to decide on a coat.

Going the other way, subtract 30 and halve. Same accuracy, same failure mode.

Why 32 and 212, and why 98.6 was never a real number

Daniel Fahrenheit built his scale around 1724 from two fixed points he could reproduce in a workshop: zero at the temperature of a brine of ice, water and ammonium chloride — the coldest thing he could reliably make — and 96 at roughly the temperature of the human body. Ninety-six because it halves cleanly six times, which is what you want when you are marking a glass tube by hand. The scale was re-anchored later on water instead, at 32 and 212, chosen so that the freezing and boiling points sit exactly 180 degrees apart. Body temperature fell out of that rework at 98.6.

That figure is a translation artefact. It comes from Carl Wunderlich's nineteenth-century German work, which put the average at 37 °C — a number with two significant figures. Convert 37 to Fahrenheit and you get 98.6, which reads like a measurement precise to a tenth of a degree and is nothing of the sort. Later datasets put the modern average closer to 36.6 °C, and healthy individuals sit anywhere across roughly a degree.

Celsius has its own oddity: Anders Celsius's original 1742 scale ran the other way up, with 0 at boiling and 100 at freezing. It was inverted after his death. The name "centigrade" was retired in 1948, which is why a weather report says degrees Celsius.

Kelvin has no degree sign

You write 300 K, not 300 °K — the degree symbol was dropped in 1967. A kelvin is exactly the same size as a Celsius degree, so the conversion is pure addition: K = C + 273.15. The reason physics uses it is that the scale starts at absolute zero, which makes ratios mean something. 600 K really is twice as hot as 300 K in the sense of carrying twice the thermal energy. 60 °C is not twice 30 °C in any sense at all, because the zero point is arbitrary.

The reference table

The oven table, which is the one people actually want

Gas marks are not a temperature scale, they are a dial with numbers on it, and the mapping below is the conventional British one. The important line is the last one: a fan or convection oven moves air, so it cooks as though it were about 20 °C hotter than the dial says. A recipe written for a conventional oven at 180 °C wants 160 °C on a fan oven, and ignoring that is why things come out dry.