What it is
The Temperature Converter works between Celsius, Fahrenheit and Kelvin.
The one conversion that is not a multiplication
Every other unit conversion is a single multiplication: a metre is always 3.28 feet, whatever the number. Temperature is not, and that is why people get it wrong.
The scales do not share a zero. Celsius puts zero at the freezing point of water; Fahrenheit puts it 32 degrees lower, at a brine freezing point Fahrenheit could reproduce in his workshop. So converting needs both a stretch and a shift:
°F = °C × 1.8 + 32, and back the other way, °C = (°F − 32) ÷ 1.8
Twenty degrees Celsius is 68 °F. Thirty is 86. And the pair worth remembering: −40 is the same number on both scales, the single point where the two lines cross.
Kelvin and absolute zero
Kelvin uses the same size of degree as Celsius but starts at absolute zero, the point where molecular motion stops. That makes converting trivial: add 273.15 to go from Celsius to Kelvin, subtract it to come back.
It exists because ratios only make sense from a true zero. Saying 200 K is twice as hot as 100 K is meaningful; saying 20 °C is twice as hot as 10 °C is not.
Ovens, weather and fever
Ovens are where this bites in daily life. 180 °C is 356 °F, which recipes round to 350 °F. The rounding is harmless for most baking but worth knowing when a recipe is fussy.
Weather needs no maths, just a couple of anchors: 0 °C is freezing and 32 °F, 20 °C is a mild day and 68 °F, 35 °C is hot and 95 °F.
Body temperature is 37 °C, which is 98.6 °F, and a fever starts around 38 °C or 100.4 °F.
How to use it
Type into any of the three boxes and the other two update at once.