Celsius to Kelvin (°C to K)
The kelvin scale starts at absolute zero, which is exactly −273.15 °C (SI Brochure, BIPM; NIST SP 811 §4.2.1.1), and its steps are the same size as Celsius degrees. To convert, add 273.15: 25 °C is 298.15 K and 100 °C is 373.15 K.
Formula
K = °C + 273.15
Example: -40 degrees Celsius = 233.15 kelvin.
Celsius to Kelvin chart
| degrees Celsius | kelvin |
|---|---|
| -273.15 | 0 |
| -196 | 77.15 |
| -100 | 173.15 |
| -40 | 233.15 |
| -18 | 255.15 |
| 0 | 273.15 |
| 10 | 283.15 |
| 20 | 293.15 |
| 25 | 298.15 |
| 30 | 303.15 |
| 37 | 310.15 |
| 50 | 323.15 |
| 100 | 373.15 |
| 200 | 473.15 |
| 500 | 773.15 |
| 1,000 | 1,273.15 |
Good to know
Gas laws need kelvin: PV = nRT and Charles's law only work with absolute temperature. Doubling a gas from 20 °C to 40 °C does not double its volume — in kelvin that is 293.15 K to 313.15 K, an increase of only 6.8%.
Standard conditions: chemistry tables use 273.15 K (0 °C) for STP and 298.15 K (25 °C) for standard thermodynamic data. Room temperature in problems is usually 293.15 K or 298.15 K.
Negative Celsius is fine, negative kelvin is not: −40 °C is 233.15 K and −196 °C (liquid nitrogen) is 77.15 K. If an answer comes out below 0 K, a sign or offset went wrong.
Frequently asked questions
How do you convert Celsius to kelvin?
Add 273.15. For example, 37 °C + 273.15 = 310.15 K.
What is 25 °C in kelvin?
25 °C is 298.15 K, the standard reference temperature in chemistry.
What is 0 °C in kelvin?
0 °C is 273.15 K, the freezing point of water.
Why is there no degree sign on K?
Since 1967 the SI unit has been the kelvin (symbol K) rather than 'degree Kelvin', so it is written without the ° symbol.