Convert Celsius to Kelvin

How to convert degrees Celsius to kelvins

Add 273.15 to the Celsius temperature: K = °C + 273.15, exactly, because the SI defines the Celsius scale as a fixed offset from the kelvin. There is no factor to memorize and nothing to round. Going back is just as direct — subtract 273.15 from a kelvin value to recover degrees Celsius.

1. Take the temperature in degrees Celsius.
2. Add 273.15.
3. The sum is the thermodynamic temperature in kelvins.

Worked example: a laboratory held at 25 °C — the reference condition behind countless data tables — sits at 25 + 273.15 = 298.15 K.

What is a degree Celsius?

The degree Celsius is the practical face of temperature measurement: weather, cooking, body temperature, and lab benches all use it. Within the SI, however, it is not an independent scale but a shifted view of the kelvin — 0 °C is defined as exactly 273.15 K — which is why converting between the two is pure addition and subtraction, and the only common temperature conversion you can do reliably in your head.

What is a kelvin?

The kelvin is the SI base unit of thermodynamic temperature, written without a degree symbol. Its zero sits at absolute zero, the coldest state physically possible, equal to -273.15 °C. Kelvin steps are the same size as Celsius degrees; only the starting point differs. The modern definition fixes the Boltzmann constant, and scientists prefer the unit because the equations of thermodynamics only behave when temperature is counted from true zero.

When you’ll need °C to K

• Gas-law problem sets. Charles’s law and the ideal gas law only work with absolute temperature: a 20 °C classroom is 293.15 K, and forgetting the offset is the single most common gas-law mistake.
• Standard conditions in chemistry. Tables quote STP at 0 °C, which is 273.15 K, while many thermodynamic data sets are tabulated at 25 °C = 298.15 K — know both, and know which one your textbook uses.
• Biology and medicine in SI form. Human body temperature of 37 °C becomes 310.15 K in any calculation that feeds physical equations, and a 60 °C water bath runs at 333.15 K.
• Cold-side physics. A -40 °C industrial freezer sits at 233.15 K, still far above absolute zero at 0 K (-273.15 °C) — kelvins make it obvious how much thermal energy remains even in things we call cold.