How this works
Kelvin and Celsius use the same degree size — the only difference is where their zeros sit. Kelvin starts at absolute zero (the lowest possible temperature, where molecular motion stops); Celsius starts at the freezing point of water. So converting is just a constant offset of 273.15. Useful in physics, chemistry, materials science, and any thermodynamic calculation where ratios of temperature need to make sense.
The formula
The 273.15 offset is the agreed temperature of the freezing point of water in kelvins (under the 1990 international temperature scale, ITS-90). To go the other way, °C → K, just add 273.15. To get Fahrenheit from Kelvin, route via Celsius: °C = K − 273.15, then °F = °C × 9/5 + 32.
Example calculation
- 0 K = −273.15 °C (absolute zero)
- 273.15 K = 0 °C (water freezes)
- 293.15 K = 20 °C (room temperature)
- 5778 K ≈ 5505 °C (the Sun's surface)
Frequently asked questions
Why no degree symbol on kelvin?
Kelvin is an SI base unit, not a "degree of" something — it stands on its own with absolute zero as a fixed origin, so you write "300 K", not "300 °K". The degree symbol is reserved for relative scales like Celsius and Fahrenheit, whose zeros are arbitrary reference points (water freezing, a brine eutectic, etc.) rather than physical absolutes. The kelvin notation was officially simplified in 1968.
Can I have a temperature below 0 K?
Classically no — 0 K is absolute zero, the temperature at which molecular motion ceases (in idealised theory; quantum mechanics says there's a residual zero-point energy). In thermodynamics it's an unreachable lower bound. There are exotic experimental systems with "negative absolute temperature" but those are population-inversion phenomena and not actually colder than 0 K — they're hotter than infinite temperature.