Subtract 273.15, that is the whole thing
The two scales run in perfect step and are offset by exactly 273.15. So 273.15 K is water's freezing point (0 C) and 373.15 K is its boiling point (100 C). For a rough figure in your head, subtracting 273 is close enough.
Kelvin starts at absolute zero, so it never goes negative
Celsius can drop below zero because it is anchored to water freezing. Kelvin is anchored to the coldest temperature that can exist, so 0 K is the floor. A negative Kelvin reading means a bad measurement, not a real temperature.
- 0 K is −273.15 C. Absolute zero, where thermal motion is at its minimum. It has never been reached; labs get within a fraction of a billionth of a Kelvin.
- The offset is exact. Use the full 273.15, not 273, for lab work. Rounding to 273 shifts every answer by 0.15.
- No degree symbol. It is 300 K, not 300°K. Celsius keeps its degree sign; Kelvin does not.
Reference points worth knowing
| Kelvin | Celsius | What it is |
|---|---|---|
| 0 K | −273.15 C | Absolute zero |
| 77.36 K | −195.79 C | Liquid nitrogen boils |
| 273.15 K | 0 C | Water freezes |
| 293.15 K | 20 C | Room temperature |
| 310.15 K | 37 C | Human body |
| 373.15 K | 100 C | Water boils (1 atm) |
| 5,778 K | 5,505 C | Sun's surface |
Science uses Kelvin because gas laws and thermodynamic equations only work with an absolute temperature. Feed Celsius into PV = nRT and the answer is wrong.
Common questions
What is the exact Kelvin to Celsius conversion?
Celsius equals Kelvin minus 273.15. The 273.15 is defined exactly by the SI system, so 273.15 K is 0 C and 300 K is 26.85 C.
Why does Kelvin have no degree sign?
Kelvin is an absolute scale, not a scale of degrees above a reference point, so you write 300 K, never 300 degrees K. Writing the degree symbol on Kelvin has been non-standard since 1967.
Is a change of 1 Kelvin the same as 1 degree Celsius?
Yes. The two scales are offset by a constant, so a difference of any size is identical in both. Heating something by 10 K is heating it by 10 C.
