Celsius to Kelvin Formula
Kelvin is just Celsius shifted by absolute zero — add 273.15.
K = °C + 273.15
How it works
Kelvin and Celsius use exactly the same size of degree, so the conversion is a shift and nothing else. What changes is where zero is put. Celsius puts zero near the freezing point of water; Kelvin puts it at absolute zero, the point at which a system holds no thermal energy that can be removed. Since 2019 the kelvin has been defined by fixing the Boltzmann constant rather than by water at all, but the 273.15 offset was chosen to keep the two scales lining up as they always had. Because the step size is identical, a temperature difference is the same number in both scales - a rise of 5 degrees C is a rise of 5 K.
Variables
- KTemperature in Kelvin (result)
- °CTemperature in Celsius (input)
Worked examples
| Input | Result |
|---|---|
| 0 °C | 273.15 K |
| −273.15 °C | 0 K (absolute zero) |
| 25 °C | 298.15 K |
Practical applications
- Gas-law calculations, which require absolute temperature and silently give nonsense if fed Celsius
- Colour temperature in lighting and photography, quoted in kelvin
- Any scientific or thermodynamic work, where kelvin is the SI unit
Common mistakes
- Rounding the offset to 273. That is fine for rough work and wrong by 0.15 K everywhere else.
- Writing 'degrees kelvin' or using a degree symbol. The unit is the kelvin and the symbol is K on its own.
- Feeding Celsius into a gas-law formula. Any calculation using absolute temperature needs kelvin, and Celsius will produce a plausible-looking but wrong answer - or a negative absolute temperature.
Use it in a tool
FAQs
Why no degree symbol on K?+
Kelvin is an absolute scale, not a 'degree' scale — so the SI standard writes K, not °K.