Fahrenheit to Celsius Converter

Temperature is unusual among conversions: scales differ not only in the size of a degree but also in where they put zero, so you can never convert one to another by multiplying alone — you always scale and then shift. Celsius sets water’s freezing point at 0 and boiling at 100; Fahrenheit puts them at 32 and 212; and Kelvin, the SI unit, starts at absolute zero (0 K = −273.15 °C = −459.67 °F) and uses no degree sign. Rankine is the absolute version of Fahrenheit, starting at the same zero as Kelvin but with Fahrenheit-sized degrees. The historical scales are still occasionally seen: Réaumur (0 to 80), Delisle (which runs backwards, 150 at freezing down to 0 at boiling), Newton (0 to 33) and Rømer (7.5 to 60). Two facts are worth remembering: absolute zero is the coldest anything can be, and −40° is the single point where Celsius and Fahrenheit read exactly the same. Oven “gas marks” are a separate cooking setting rather than a true scale (roughly gas mark = (°F − 250) ÷ 25), so they are not included here.