Temperature converter

Because the scales start at different places. Zero kilograms is zero pounds, but 0 °C is 32 °F: the two scales disagree about where zero is as well as about the size of a degree. A conversion therefore scales and shifts. Twenty-five degrees Celsius to Fahrenheit: 25 × 9/5 = 45, and 45 + 32 = 77 °F. Back again: (77 − 32) × 5/9 = 25 °C.

The two scales cross once, at −40, where Celsius and Fahrenheit read the same number.

Without the 32. A rise of 10 °C is a rise of 18 °F, because a difference between two temperatures has no zero point to shift — only the size of the degree matters. This is the most common temperature mistake: a forecast that says it will be 10 degrees warmer tomorrow in Celsius means 18 degrees warmer in Fahrenheit, not 50.

Daniel Gabriel Fahrenheit published his scale in 1724. Anders Celsius proposed his in 1742 upside down, with 100 for freezing water and 0 for boiling; the order was reversed around the time of his death in 1744. The kelvin, the SI unit, starts at absolute zero, −273.15 °C, the point at which nothing can be colder. Since 2019 it has been defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K, and since 1967 it has been written without a degree sign: 300 K, not 300 °K.

The familiar 98.6 °F looks oddly precise for a reason: it is simply 37 °C converted, carrying a decimal the original round number never had.

Only at standard sea-level pressure. Boiling point falls as air pressure drops, so in a city at 2,000 m of altitude water boils at roughly 93 °C, and food in boiling water cooks more slowly. For the two directions people search most, use Celsius to Fahrenheit and Fahrenheit to Celsius.