How to Convert Celsius to Kelvin
Converting Celsius to Kelvin is the simplest unit conversion in all of science. The Celsius and Kelvin scales share the exact same degree size — one Kelvin equals one degree Celsius. The only difference is where zero sits: Celsius places zero at water's freezing point; Kelvin places zero at absolute zero, the theoretical temperature at which all molecular motion stops. This means the conversion is a pure offset: add 273.15. No multiplication, no fractions — just a fixed shift. This page uses the internationally accepted definition: K = °C + 273.15.
The Conversion Formula
K = °C + 273.15
Worked Examples
Example 1: Convert 0°C (water freezing point)
0 + 273.15 = 273.15 K — the ice point in thermodynamic temperature
Example 2: Convert 100°C (water boiling point)
100 + 273.15 = 373.15 K — standard boiling point at 1 atm
Example 3: Convert -273.15°C (absolute zero)
-273.15 + 273.15 = 0 K — the coldest possible temperature in the universe
Common Celsius to Kelvin Conversions
| °C | K | Notes |
|---|---|---|
| -273.15°C | 0 K | Absolute zero |
| -196°C | 77.15 K | Liquid nitrogen boiling point |
| -78°C | 195.15 K | Dry ice sublimation point |
| 0°C | 273.15 K | Water freezes |
| 20°C | 293.15 K | Room temperature (STP) |
| 25°C | 298.15 K | Standard laboratory temperature |
| 37°C | 310.15 K | Human body temperature |
| 100°C | 373.15 K | Water boils |
| 5,500°C | 5,773.15 K | Surface of the Sun (approx.) |
Related Unit Converters
Frequently Asked Questions
Why is the Kelvin scale important?
Kelvin is the SI base unit of thermodynamic temperature. Unlike Celsius and Fahrenheit, it starts at absolute zero — the theoretical point where all thermal motion ceases. This makes it essential for all scientific calculations involving temperature ratios, particularly in thermodynamics, gas laws (PV = nRT), blackbody radiation, and cryogenics. You cannot use Celsius or Fahrenheit in any equation where temperature appears in a denominator or exponent — only Kelvin works.
Why 273.15? Where does that number come from?
The 273.15 offset is exact by definition since 1954. The triple point of water (where ice, liquid water, and water vapor coexist in equilibrium) was assigned the value 273.16 K. Combined with the definition that 1 K = 1°C in magnitude, the freezing point of water at standard atmospheric pressure falls at exactly 273.15 K. The value is a defined constant, not a measurement — it cannot change.
Is there a temperature below 0 K?
No. 0 K (absolute zero) is the theoretical lower bound of temperature. At this point, a system is in its ground state with minimum possible energy. However, physicists have created systems with negative Kelvin temperatures in the laboratory — these are not "colder than absolute zero" but rather hotter than any positive temperature, occurring in systems with a population inversion (like a laser medium) where adding energy decreases entropy.
Engineering Context
The Kelvin scale is mandatory in every engineering discipline where temperature ratios matter. In gas turbine design, compressor pressure ratios depend on absolute temperature — using Celsius instead of Kelvin in a Brayton cycle calculation gives nonsensical efficiency numbers. In cryogenic engineering, liquid hydrogen is stored at 20 K (-253.15°C); a 1 K error in a valve spec means the difference between a liquid and a gas. In semiconductor manufacturing, dopant diffusion rates follow an Arrhenius relationship exp(-Ea/kT) — the T must be in Kelvin, or the entire process model is garbage. The Celsius-to-Kelvin conversion (K = °C + 273.15) is exact by definition under the 2019 SI redefinition, which ties the kelvin to the Boltzmann constant k = 1.380649 × 10⁻²³ J/K. There is no approximation, no rounding, and no tolerance — adding 273.15 is the complete and exact conversion.
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