Convert °C to °F

Celsius puts zero at the freezing point of water and 100 at its boiling point, at sea level. Anders Celsius originally had the scale the other way up, with zero as boiling.

Fahrenheit puts freezing at 32 and boiling at 212, so a Fahrenheit degree is five ninths the size of a Celsius one. Converting therefore takes two steps, not one: a multiplication for the difference in degree size, and a shift for the different zero points.

Celsius and Fahrenheit do not start counting from the same place, so no single number converts between them. Multiplying is the mistake this page exists to prevent: it is right at exactly one temperature and wrong everywhere else, and the error grows the further you get from that point.

The two ends of this page's own scale show it. 0 °C is 32 °F and 100 °C is 212 °F — a hundred steps on one side, 180 on the other, and the two zeros in different places. That is why the formula above has two parts, something to multiply by and something to add; drop the addition and the answer stops being wrong slowly and starts being wrong badly.

-40 °C and -40 °F are the same temperature — the one reading that needs no conversion at all. It is the quickest proof that the scales are offset rather than scaled: if a factor were all that separated them, they would only ever agree at zero.

It is also a useful anchor going the other way. Any temperature you convert should land on the far side of -40° from where it started, and if it does not, the addition went in the wrong direction.

The Celsius weather range is short enough to learn outright, which beats converting each figure. 0 °C is freezing, 10 °C wants a jacket, 20 °C is a pleasant day, 30 °C is hot, 40 °C is a heat emergency. In Fahrenheit those are 32, 50, 68, 86 and 104 — five landmarks that cover almost every forecast you will read on a trip.

The compression is what takes getting used to. One Celsius degree is 1.8 Fahrenheit degrees, so a forecast moving from 22 to 26 has moved from 72 to 79, and a range that looks narrow is not. It is also why metric forecasts quote fewer decimals: a single Celsius degree carries nearly as much information as two Fahrenheit ones, and the extra digit would be noise.

The mental version is to double the Celsius figure and add 30. 20 °C gives 70 against 68 exactly; 25 °C gives 80 against 77; 15 °C gives 60 against 59. The rule is exact at 10 °C and drifts by 0.2 °F for every degree away from it, so it runs about four degrees high at 30 °C and four low at −10 °C — close enough to decide what to pack.

It collapses on anything hot, because the error keeps growing: 180 °C comes out as 390 against a true 356, and 200 °C as 430 against 392. For an oven, an engine or a datasheet — anything above roughly 40 °C — do the real arithmetic, multiply by 1.8 and add 32, or read the value out of the table above.

180 °C is 356 °F and the dial you set is 350. Metric recipes step in twenties — 160, 180, 200, 220 °C — which land on 320, 356, 392 and 428 °F against American marks at 325, 350, 400 and 425. Every pair is within eight degrees, comfortably inside the swing of a domestic oven, so the rounding never needs a thought.

What does need a thought is the fan. Where a European recipe specifies a fan oven — it may say fan, convection, or print both numbers — the equivalent conventional setting is about 20 °C higher. So 160 °C fan is 180 °C conventional, or 350 °F. Convert the fan number straight across and set a conventional American oven to it and the oven runs some 35 °F cool, which shows up as a longer bake and a pale top.

An American freezer is set to 0 °F and a European one to −18 °C, and those are the same instruction: −18 °C is −0.4 °F. Both descend from the same food-safety recommendation, expressed as a round number in the local scale. The refrigerator pair works out the same way — 4 °C against 40 °F — so a European fridge specification needs no conversion at all to act on.

Appliance and storage specifications are one of the few places a metric temperature reaches an American reader with a decision attached, so they are worth being able to read. A wine cabinet rated 5 to 18 °C runs 41 to 64 °F. A chest freezer claiming −24 °C reaches −11 °F. The cold-chain range of 2 to 8 °C, which is how vaccines and many medicines are specified worldwide, is 36 to 46 °F.

−40 °C is −40 °F. It is the one reading that is the same number on both scales, and it falls straight out of the arithmetic: the two lines have different slopes, so they cross exactly once, and −40 is where. Above that point the Fahrenheit figure is the larger of the pair; below it the Celsius figure is.

It is a genuinely useful check rather than a curiosity. Any conversion in either direction should land on the correct side of −40, and any remembered shortcut that puts it on the wrong side has been remembered wrong. It also explains why cold-weather ratings for fuel, lubricants, batteries and equipment are so often quoted at −40 with no scale attached: at that single temperature the label does not need one.

Climate targets are where this goes wrong most publicly. A 1.5 °C rise is 2.7 °F, not 34.7 °F. A rise is a difference, and a difference is multiplied by 9/5 with nothing added, because the 32 in the formula is the distance between two zeros and a difference does not have a zero in it. The same figure run through the full formula produces a mild spring morning.

It applies anywhere a Celsius number describes a change rather than a state. A thermostat turned up 2 °C has been turned up 3.6 °F. A motor rated for a 40 °C rise above ambient is rated for 72 °F. A tolerance of ±0.5 °C is ±0.9 °F. The word in front of the number decides it: "is" takes the full formula, while "warmer", "rise", "per" and "±" take the 9/5 alone.

Anders Celsius published his scale in 1742 with the numbers inverted: 0 was the boiling point of water and 100 its freezing point, so a colder day carried the larger number. It was turned the right way up within a few years of his death in 1744, and the version the world uses is the reversed one rather than the one he wrote.

The name took two centuries longer to settle. English called it centigrade well into the twentieth century, and the change to "degree Celsius" was made formally in 1948; French and Spanish speakers still reach for centigrade and centígrado in conversation. Part of the reason for tidying the name was that a centigrade already meant something else — a hundredth of a grade in angular measure — which is a collision nobody wanted in a technical document.

A fever threshold quoted abroad will be 38 °C, which is 100.4 °F — the same number an American thermometer prints, arrived at from the other side. 37 °C is 98.6 °F, 39 °C is 102.2 °F, and 40 °C, the point at which most guidance says to seek help, is 104 °F. None of the Fahrenheit figures is round, because none of them was chosen in Fahrenheit.

The step size matters when you are reading foreign medical instructions. One Celsius degree spans nearly two Fahrenheit degrees, so advice written in whole Celsius degrees is coarser than it looks, and the halves are doing real work: "above 38.5" means above 101.3 °F, nearly a full Fahrenheit degree past the 38 threshold. A digital thermometer set to Celsius shows a decimal for exactly this reason.