Convert Celsius to Kelvin
Celsius to Kelvin with the exact 273.15 offset, absolute zero marked, Fahrenheit alongside and a slider from absolute zero to 1000 °C.
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The offset is exactly 273.15 — no multiplication involved.
K = °C + 273.15 K = 25 + 273.15 = 298.15 K
| degrees Celsius | kelvin |
|---|---|
| −273.15 °C | 0.00 K |
| −200 °C | 73.15 K |
| −100 °C | 173.15 K |
| −40 °C | 233.15 K |
| 0 °C | 273.15 K |
| 10 °C | 283.15 K |
| 20 °C | 293.15 K |
| 25 °C | 298.15 K |
| 37 °C | 310.15 K |
| 100 °C | 373.15 K |
| 200 °C | 473.15 K |
| 500 °C | 773.15 K |
| 1,000 °C | 1,273.15 K |
Same degree, different zero
Kelvin and Celsius are the only pair of temperature scales that share a degree size. One kelvin is exactly one degree Celsius; the two scales differ only in where they put zero. So the conversion is a single addition:
K = °C + 273.15
No multiplication, no fractions. 0 °C is 273.15 K, 25 °C is 298.15 K, and 100 °C is 373.15 K.
Why 273.15 and not 273
Because the kelvin was originally defined by placing the triple point of water — the unique temperature and pressure at which ice, liquid water and vapour coexist — at exactly 273.16 K. The ordinary ice point sits 0.01 K below that, at 273.15 K, and that is the number Celsius calls zero. Since the 2019 SI redefinition the kelvin is fixed instead by the Boltzmann constant, but the numbers were chosen so nothing changed in practice.
Dropping the 0.15 is fine in a school exercise and wrong in thermodynamics, where absolute temperatures appear in ratios and a 0.05% error propagates.
Writing it correctly
- Write 300 K, not 300 °K. Kelvin is not a degree scale.
- Say "three hundred kelvin", not "three hundred degrees kelvin".
- The unit symbol is a capital K; the unit name, when written out, is lower case.
- Temperature differences can be given in either K or °C interchangeably, since the degree size is identical.
Reference points on the absolute scale
| Kelvin | Celsius | What happens there |
|---|---|---|
| 0 | -273.15 | Absolute zero — unreachable, only approachable |
| 2.7 | -270.45 | The cosmic microwave background |
| 77 | -196.15 | Liquid nitrogen boils |
| 273.15 | 0 | Ice melts |
| 293.15 | 20 | Room temperature |
| 310.15 | 37 | Human body temperature |
| 373.15 | 100 | Water boils at sea level |
| 1811 | 1537.85 | Iron melts |
| 5772 | 5498.85 | The Sun's photosphere |
Where kelvin is compulsory
Anywhere temperature enters a physical law as a ratio or a product: the ideal gas law, the Stefan–Boltzmann law, blackbody spectra, reaction-rate equations, noise figures in electronics. Doubling the absolute temperature of a gas at constant pressure doubles its volume; doubling the Celsius temperature does nothing predictable at all, because the Celsius zero is arbitrary.
The other place you meet kelvin daily is lighting: a "2700 K" bulb is warm white and a "6500 K" bulb is daylight. That is a colour temperature — the temperature of a blackbody radiating that colour — not the temperature of the bulb.
Questions
What is the Celsius to Kelvin formula?
K = °C + 273.15. There is no multiplication: a kelvin and a degree Celsius are exactly the same size, so only the zero point moves.
Why is it 273.15 and not 273?
Because the kelvin is defined so that the triple point of water sits at 273.16 K, which puts the ice point at 273.15 K. Dropping the 0.15 is a common shortcut in school problems and a real error in thermodynamics.
Do you write "degrees Kelvin"?
No. Kelvin is not a degree scale — it is written 300 K, said "three hundred kelvin", with no degree sign and no capital on the unit name.
What is absolute zero?
0 K, which is −273.15 °C or −459.67 °F. It is the point at which a system has minimum possible thermal energy, and it cannot be reached, only approached.