Fahrenheit to Celsius
Subtract 32 first, then divide by 1.8. Room temperature at 72 °F is 22.2 °C, a 350 °F oven is 176.7 °C, and 98.6 °F is exactly 37 °C. Doing the two operations in the other order produces an answer that is 14.2 degrees too low, at every single temperature.
How to convert °F to °C
Ovens, and the two roundings that follow
American recipes cluster on multiples of 25 °F, and none of them convert to round Celsius numbers. 325, 350, 375, 400 and 425 °F are 162.8, 176.7, 190.6, 204.4 and 218.3 °C. European ovens are marked in twenties, so those become 160, 180, 190, 200 and 220 in practice, and every one of those is a rounding rather than the answer.
A fan oven then wants a second adjustment that has nothing to do with the conversion at all. Moving air transfers heat to the food faster, so the usual guidance is to drop the dial by around 20 °C or to shorten the time instead. A 350 °F recipe therefore lands at 180 °C conventional and roughly 160 °C fan, which is a long way from where arithmetic alone would put it.
British gas marks are a third scale, and they are defined cleanly against Fahrenheit rather than against Celsius. Gas mark 1 is 275 °F, and each mark up adds 25 °F. That puts gas mark 4 at 350 °F, which is the 180 °C most recipes call a moderate oven, and gas mark 7 at 425 °F, or 220 °C.
The order of the two operations
Subtract, then divide. Dividing first turns 212 °F into 85.8 instead of 100 and 32 °F into −14.2 instead of 0. What makes this one hard to catch is that the error is a flat 14.2 degrees everywhere, so the answers stay plausible and internally consistent; they are simply all wrong by the same amount. The formula is written °C = (°F − 32) ÷ 1.8 with the brackets for exactly this reason.
Where the scale came from
Daniel Gabriel Fahrenheit set his zero at the temperature of a brine of ice, water and ammonium chloride, and put 96 at roughly the heat of the human body. Freezing and boiling water landed at 32 and 212 as consequences rather than as choices. The scale was later redefined onto those two points, because they are far easier to reproduce in a laboratory than a particular bucket of brine, and that redefinition is why body temperature no longer sits at the round 96 he intended.
What people use it for
- Baking an American recipe in a European oven
- Reading a US weather forecast before travelling
- Setting a thermostat or air conditioner labelled in Fahrenheit
- Turning an American gas mark or oven dial into a Celsius setting
- Converting a meat probe or sugar-work target into Celsius
- Reading an operating range on a datasheet written in Fahrenheit
Questions
176.67 °C. Recipes round it to 180 °C, or about 160 °C in a fan oven.
204.4 and 218.3 °C, written as 200 and 220 on a European dial.
350 °F, so 176.7 °C, usually set as 180. Gas mark 1 is 275 °F and every mark above it adds 25 °F.
For weather, yes: 72 °F gives 21 against a true 22.2. The error grows at the extremes, so do not bake with it.
Because dividing before subtracting scales the 32 as well, which leaves every answer 14.2 degrees low. Nothing about the result looks wrong, which is what makes it worth checking.
0 °F is the American convention, which is −17.8 °C. European freezers are marked −18, which is the same setting rounded in the other unit.
Exactly 37 °C. The oddness is in the Fahrenheit figure, which is a converted round number rather than a measured one.
100.4 °F is 38 °C, the usual clinical threshold. This page does arithmetic and is not medical advice.
Convert each end separately: 20.0 to 22.2 °C. A tolerance is different. ±2 °F is a difference, so it is only divided by 1.8, giving ±1.11 °C.
−40 °C. It is the one temperature where the two scales read the same, which is why it appears without a unit in cold-weather specifications.
Because his reference points were a brine mixture and body heat, not water. Water’s freezing point simply fell at 32 on the scale he had already drawn.
No. The arithmetic runs at full precision and rounds once for display. The decimals row changes what is shown and nothing else.