Converters
Celsius to Kelvin
The formula is K = °C + 273.15. Unlike the Celsius–Fahrenheit conversion, this one is just addition — the two scales use identical step sizes. Below: why Kelvin exists, where it's used, and a full reference chart.
The formula
K = °C + 273.15
To go back: °C = K − 273.15
Kelvin and Celsius are the same scale shifted by 273.15. A temperature change of 1°C is exactly a change of 1 K. The only difference is where zero sits: Celsius puts zero at the freezing point of water, Kelvin puts zero at absolute zero — the coldest anything can be.
Why Kelvin exists
In the 19th century, physicists studying gases noticed that gas volume decreases linearly as temperature drops. Extrapolating the trend pointed to a theoretical minimum — a temperature where all molecular motion would stop. Lord Kelvin calculated this to be −273°C.
This became the basis for an absolute scale. Starting at zero where physics actually bottoms out — rather than at an arbitrary reference like water's freezing point — makes equations far cleaner. The ideal gas law PV = nRT, for example, requires T to be in Kelvin. If you plug in Celsius, you get wrong answers below 0°C because negative temperatures produce negative volumes, which are nonsensical.
Kelvin is also used in thermodynamics, astrophysics, and cryogenics — anywhere temperature ratios matter, not just differences. Saying the sun is "27 times hotter than room temperature" only makes sense in Kelvin (5778 K vs 298 K = 19×, which gives the right ratio). In Celsius, the ratio doesn't mean anything physically.
Worked examples
0°C→273.15 KWater freezes25°C→298.15 KStandard lab temperature100°C→373.15 KWater boilsAbsolute zero
Absolute zero is 0 K = −273.15°C. No system can reach exactly absolute zero — the third law of thermodynamics says it would take infinite energy to get there. But physicists have gotten extremely close. In 2021, a research team in Germany cooled rubidium atoms to 38 picokelvin (0.000000000038 K) — the coldest temperature ever recorded.
At these temperatures, quantum effects dominate completely. Atoms enter states called Bose-Einstein condensates, where they behave as a single quantum entity. This matters for superconductor research, quantum computing, and fundamental physics experiments.
Celsius to Kelvin reference chart
| °C | K | Context |
|---|---|---|
| -273.15 | 0 | Absolute zero |
| -196 | 77.15 | Liquid nitrogen |
| -78.5 | 194.65 | Dry ice |
| 0 | 273.15 | Water freezes |
| 20 | 293.15 | Room temperature |
| 25 | 298.15 | Standard temp (science) |
| 37 | 310.15 | Body temperature |
| 100 | 373.15 | Water boils |
| 1000 | 1273.15 | Lava (approx) |
| 5500 | 5773.15 | Sun's surface (approx) |
Frequently Asked Questions
What is the formula to convert Celsius to Kelvin?
Add 273.15 to the Celsius temperature. Written out: K = °C + 273.15. For example, 25°C = 25 + 273.15 = 298.15 K. The conversion is simply a shift — Kelvin and Celsius use the same step size, just different zero points.
What is 0°C in Kelvin?
0°C is 273.15 K — the freezing point of water on the Kelvin scale. This is also why Kelvin uses 273.15 as its offset: 0°C corresponds to 273.15 K because absolute zero (0 K) is at −273.15°C.
What is absolute zero in Celsius?
Absolute zero is −273.15°C, or 0 K. It is the lowest theoretically possible temperature — the point where particles have minimum thermal motion. It has never been fully achieved in practice, though scientists have come within billionths of a degree.
Why is Kelvin used in science instead of Celsius?
Many physics equations — including the ideal gas law (PV = nRT), the Stefan–Boltzmann law, and Wien's displacement law — require absolute temperature values. Kelvin starts at absolute zero with no negative values, so these equations work correctly. If you used Celsius, temperatures below 0°C would produce nonsensical negative results in equations that expect positive absolute values.
Does Kelvin use degrees like Celsius?
No. Kelvin is written without a degree symbol. You write 300 K, not 300°K. The 'degree' notation was officially dropped in 1968 by the International Bureau of Weights and Measures (BIPM) to reflect that Kelvin is an absolute scale, not a relative one.
What is room temperature in Kelvin?
Standard room temperature is approximately 293–298 K. In scientific contexts, 'room temperature' is often defined as exactly 298.15 K (25°C). In physics, 'standard temperature' for the ideal gas law is 273.15 K (0°C) at 1 atmosphere pressure.