Temperature converter
Convert between Celsius, Fahrenheit and Kelvin. Formulas, reference points, and why temperature needs an offset rather than a factor.
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Why temperature is not like other conversions
Every other converter here multiplies by a factor. Temperature cannot, because the three scales disagree about where zero sits. Converting needs a ratio and an offset:
°F = (°C × 9/5) + 32
°C = (°F - 32) × 5/9
K = °C + 273.15The practical consequence catches people out constantly: a difference of 1 °C is 1.8 °F, but a reading of 1 °C is 33.8 °F. If you are converting a temperature change, use the ratio alone. If you are converting a temperature, use the whole formula.
Reference points
| Celsius | Fahrenheit | Kelvin | |
|---|---|---|---|
| -273.15 | -459.67 | 0 | absolute zero |
| -40 | -40 | 233.15 | the scales cross |
| 0 | 32 | 273.15 | water freezes |
| 20 | 68 | 293.15 | room temperature |
| 37 | 98.6 | 310.15 | body temperature |
| 100 | 212 | 373.15 | water boils at sea level |
Kelvin has no degrees
Kelvin is written 300 K, not 300 °K. It is an absolute scale: zero really is the
bottom, the point at which a system has no thermal energy left to remove. Celsius and
Fahrenheit both place zero at an arbitrary point, which is why negative values are ordinary
in one and meaningless in the other.
Because Kelvin and Celsius share the same degree size, converting between them is the one temperature conversion that is pure addition.
Boiling point depends on where you are
The 100 °C figure holds at sea level. Water boils when its vapour pressure matches the surrounding air pressure, so at altitude, where the air pushes down less, it boils cooler: roughly 93 °C in Denver, and about 71 °C at Everest base camp. This is why high-altitude recipes call for longer cooking times rather than higher settings.