Celsius to Fahrenheit
Multiply by 1.8 and add 32. That addition is what makes temperature unlike every other conversion on this site: the two scales do not share a zero, so there is no single number to multiply by. Water freezes at 0 °C or 32 °F, boils at 100 °C or 212 °F, and the two scales cross at −40.
How to convert °C to °F
The offset is also a trap, and it catches people converting the wrong kind of number. A temperature takes the full formula. A temperature difference takes only the multiplication. If a fridge warms by 10 °C it has warmed by 18 °F, not by 50. Anything phrased as a rise, a drop, a range width, a tolerance or a gradient is a difference, so leave the 32 out of it: a spec sheet reading ±2 °C is ±3.6 °F.
Anders Celsius published his scale in 1742 the other way up. He put zero at the boiling point of water and 100 at the freezing point, so his numbers rose as things got colder. It was inverted within a few years of his death and then spent two centuries being called centigrade. The ninth General Conference on Weights and Measures adopted the name Celsius in 1948, partly to honour him and partly because in several countries a centigrade already meant a hundredth of a grade, which is a unit of angle.
The two scales also carry different resolutions in the range people live in. A Fahrenheit degree is 0.56 °C, so a forecast given in Fahrenheit is nearly twice as fine-grained without needing a decimal point, which is part of why American weather reporting rarely uses one. The band most people call comfortable, 15 to 25 °C, runs from 59 to 77 °F.
What people use it for
- Reporting a 38.5 °C fever to a US clinic that reads from 98.6 °F
- Putting a −20 to 60 °C storage range on a spec sheet for a US customer
- Giving an aquarium or hot-tub target of 28 °C to an American reader
- Quoting a 40 °C European heatwave record for a US audience
- Converting a tolerance rather than a temperature
Questions
Double the Celsius and add 30. Twenty becomes 70 against a true 68, and 30 becomes 90 against 86. It runs high, and the error grows with the temperature.
At −40. Minus forty Celsius is minus forty Fahrenheit, the single point where the two lines cross.
Multiply by 1.8 and stop. A ten degree Celsius rise is an eighteen degree Fahrenheit rise. Adding 32 to a difference is the commonest error on this conversion.
0 °C is 32 °F, 10 is 50, 20 is 68, 25 is 77, 30 is 86, 35 is 95 and 40 is 104.
Because it is the conversion of a round 37 °C, taken from Carl Wunderlich’s nineteenth-century study. Modern measurements put the average slightly lower, nearer 36.6 °C.
For practical purposes yes. The scale was renamed in 1948 to avoid a clash with the centesimal angular unit of the same name.
Kelvin uses the same degree size and starts at absolute zero, so K = °C + 273.15. Since 2019 it has been defined by fixing the Boltzmann constant at 1.380649 × 10⁻²³ joules per kelvin.
Because 212 minus 32 is 180. Fahrenheit fixed his reference points elsewhere and those two landed where they did as a consequence; the fact that 180 divides neatly by 2, 3, 4, 5, 6 and 9 is a happy accident rather than a design.