How temperature conversion works
Fahrenheit to Celsius, Celsius to Fahrenheit, and Kelvin are all connected by two simple linear formulas. Celsius and Kelvin use the same step size and differ only by an offset of 273.15; Fahrenheit uses smaller degrees (a Fahrenheit degree is 5⁄9 of a Celsius degree) and a different zero point. Enter any one of the three and this converter returns the other two — and warns you if the value falls below absolute zero, the hard floor of temperature itself.
The formulas
Here °F is degrees Fahrenheit, °C is degrees Celsius, and K is kelvin — no degree sign, because since 1967 the kelvin has been a plain SI unit, so it's "310.15 K", never "degrees Kelvin".
Worked examples
°F → °C: body temperature, 98.6 °F: (98.6 − 32) × 5⁄9 = 66.6 × 5⁄9 = 37 °C (310.15 K).
°C → °F: a mild day, 20 °C: 20 × 9⁄5 + 32 = 36 + 32 = 68 °F (293.15 K).
K → °C and °F: 300 K: 300 − 273.15 = 26.85 °C, and 26.85 × 9⁄5 + 32 = 80.33 °F.
Reference points on all three scales
| Fahrenheit | Celsius | Kelvin | |
|---|---|---|---|
| Absolute zero | −459.67 °F | −273.15 °C | 0 K |
| Water freezes | 32 °F | 0 °C | 273.15 K |
| Body temperature | 98.6 °F | 37 °C | 310.15 K |
| Water boils | 212 °F | 100 °C | 373.15 K |
Where the scales come from — and the −40 trick
Fahrenheit set his zero around 1724 at the coldest temperature he could reliably reproduce: an ice, water, and ammonium chloride brine. His scale's upper anchor was near human body temperature, originally pegged at 96. Celsius built his scale in 1742 on water's freezing and boiling points, 100 degrees apart — though his original ran backwards, with 0 at boiling and 100 at freezing, and was flipped shortly after his death. Kelvin simply slides the Celsius scale down so that 0 sits at absolute zero, the point where no more thermal energy can be removed.
Because the two everyday scales are straight lines with different slopes, they cross exactly once: at −40, where −40 °F and −40 °C are the same temperature. And for mental conversions, Celsius to Fahrenheit is roughly "double it and add 30" — exact at 10 °C and within a few degrees for most weather.