Celsius To Kelvin Converter
There is a coldest possible temperature, and the Kelvin scale starts exactly there. Convert any Celsius reading to kelvin and you are measuring how far it sits above that absolute floor, which is why physicists, chemists and engineers reach for this conversion all the time.
Quick answer: add 273.15 to the Celsius temperature. So 0 °C is 273.15 K, 25 °C is 298.15 K and 100 °C is 373.15 K.
The Celsius to Kelvin formula
This is the simplest temperature conversion there is, because the two scales use degrees of exactly the same size. Only the starting point differs.
NIST confirms the key idea: a change of one kelvin is the same amount of temperature change as one degree Celsius. The scales differ only in where zero sits. Kelvin zero is absolute zero, while Celsius zero is the freezing point of water.
What the kelvin actually is
The kelvin, symbol K, is the SI unit of thermodynamic temperature. Since the SI revision approved in November 2018, its size is fixed by giving the Boltzmann constant the exact value 1.380649 × 10−23 joules per kelvin (NIST). Before that, the kelvin was defined as 1/273.16 of the thermodynamic temperature of the triple point of water, the unique condition where ice, liquid water and vapour coexist, which sits at 273.16 K (NIST).
Notice the notation: it is written as 300 K, not 300 °K. The kelvin takes no degree sign.
How to convert Celsius to Kelvin, step by step
- Write down the temperature in °C.
- Add 273.15.
- Write the result in K, without a degree symbol.
Example 1: room temperature
- Start with 21.5 °C.
- 21.5 + 273.15 = 294.65.
21.5 °C = 294.65 K
Example 2: boiling water
- Start with 100 °C.
- 100 + 273.15 = 373.15.
100 °C = 373.15 K
Example 3: a negative Celsius value
- Start with −40 °C.
- −40 + 273.15 = 233.15.
−40 °C = 233.15 K
The answer is always positive for any physically possible temperature, because nothing can be colder than −273.15 °C.
Celsius to Kelvin conversion table
| Celsius (°C) | Kelvin (K) |
|---|---|
| -273.15 °C | 0 K |
| -196 °C | 77.15 K |
| -78.5 °C | 194.65 K |
| -40 °C | 233.15 K |
| -20 °C | 253.15 K |
| 0 °C | 273.15 K |
| 0.01 °C | 273.16 K |
| 20 °C | 293.15 K |
| 25 °C | 298.15 K |
| 37 °C | 310.15 K |
| 100 °C | 373.15 K |
| 1000 °C | 1273.15 K |
Why scientists use kelvin
Many physical laws, such as the ideal gas law and the Stefan-Boltzmann law for thermal radiation, only work when temperature is measured from absolute zero. Using Celsius there would give nonsense, such as a gas at 0 °C having zero volume.
Kelvin has no negative numbers, so ratios behave sensibly: 400 K really is twice as hot as 200 K in terms of thermal energy, while 40 °C is not twice 20 °C. That is why chemistry, physics and engineering formulas almost always ask for kelvin.
Precision: 273.15 or 273?
School problems sometimes round the offset to 273, which is fine for a quick estimate. For anything you will publish or build, keep 273.15. A reading of 25 °C is 298.15 K, and rounding the offset would shift it by 0.15 K, a difference that can matter in calibration, climate data and laboratory work.
Common mistakes
- Writing °K. The correct symbol is K.
- Forgetting the decimals. The offset is 273.15, not 273.
- Converting differences. A temperature change in °C is the same number in K. Do not add 273.15 to a difference; use the temperature difference calculator.
Related converters
Reverse the process with Kelvin to Celsius, or bring in Fahrenheit through Celsius to Fahrenheit and Fahrenheit to Kelvin. The absolute temperature calculator converts one value into every absolute scale at once.
Everyday temperatures in kelvin
It helps to build a feel for kelvin numbers. A chilly 5 °C day is 278.15 K, a comfortable 20 °C room is 293.15 K, and a hot 40 °C afternoon is 313.15 K. The numbers look large because kelvin counts from absolute zero, but the gaps between them are exactly the same as on the Celsius scale: 20 °C and 25 °C differ by 5 degrees, and so do 293.15 K and 298.15 K.
Converting a whole list of readings
If you are converting many readings, such as a column of sensor data, the rule never changes: add 273.15 to every value. In a spreadsheet that is a single formula, for example =A2+273.15, filled down the column. Keep the original Celsius column so you can audit the results later, and format the kelvin column with the same number of decimals as the source data.
Celsius, Kelvin and the ideal gas law
The ideal gas law, PV = nRT, is the classic reason to convert. The T must be in kelvin, because the law relates pressure and volume to the average thermal energy of the molecules, which is zero at absolute zero.
Feeding Celsius into the equation would make a gas at 0 °C appear to have no pressure or volume. Converting first, as in 25 °C to 298.15 K, keeps the equation honest.
Frequently asked questions
What is the formula for Celsius to Kelvin?
Add 273.15 to the Celsius temperature: K = °C + 273.15.
What is 0 °C in Kelvin?
0 °C equals 273.15 K, the freezing point of water.
What is absolute zero in Celsius?
Absolute zero is 0 K, which equals −273.15 °C.
Do Celsius and Kelvin degrees have the same size?
Yes. A change of one kelvin is the same amount of temperature change as one degree Celsius (NIST).
Why is Kelvin written without a degree sign?
The kelvin is an SI base unit with its own name, so the symbol is simply K, as in 300 K.
Can a Kelvin temperature be negative?
No. Kelvin starts at absolute zero, so every physically possible temperature is zero or positive.
Sources
- NIST: Kelvin, introduction to the redefined SI unit (nist.gov)
- CalculatorSoup: Celsius to Kelvin (calculatorsoup.com)
- Britannica: Celsius temperature scale (britannica.com)
