Kelvin and Celsius are directly related by a simple offset: the Kelvin scale is exactly 273.15 units higher than the Celsius scale. To convert, you add 273.15 to any Celsius temperature to get Kelvin, or subtract 273.15 from Kelvin to get Celsius. The size of one degree is identical on both scales, so a change of 1 °C is the same as a change of 1 K.
The Conversion Formula
The math here is as simple as temperature conversions get:
- Celsius to Kelvin: K = °C + 273.15
- Kelvin to Celsius: °C = K − 273.15
There’s no multiplication or division involved, unlike converting between Fahrenheit and Celsius. You’re just shifting the number up or down by 273.15. Water freezes at 0 °C, which is 273.15 K. Water boils at 100 °C, which is 373.15 K. The 100-unit gap between freezing and boiling is the same on both scales.
Why the Scales Share the Same Degree Size
Both scales use the same interval for one degree. As the National Institute of Standards and Technology puts it, “One Celsius degree is an interval of 1 K.” If a room warms up by 5 degrees Celsius, it also warms up by 5 Kelvin. This is what makes converting between the two so straightforward: the only difference is where zero sits.
On the Celsius scale, zero is pegged to the freezing point of water. On the Kelvin scale, zero is set at absolute zero, the coldest temperature theoretically possible. That’s the point at which matter has essentially no thermal energy left to give up, and it falls at −273.15 °C.
What Absolute Zero Means
In the 1800s, scientists studying how gases behave at different temperatures noticed a pattern: as a gas cools, its volume shrinks in a predictable, straight-line fashion. If you extend that line far enough, the volume would theoretically reach zero at −273.15 °C. Lord Kelvin used this insight to propose an absolute temperature scale starting at that point. On his scale, 0 K represents a true physical floor, not an arbitrary marker like the freezing point of water.
Nothing can be cooled below 0 K. Laboratories have reached temperatures within a tiny fraction of a degree above absolute zero, but never quite hit it. This lower limit is baked into the laws of physics, which is exactly why scientists wanted a scale that starts there.
Everyday Reference Points in Both Scales
Seeing a few familiar temperatures side by side makes the relationship concrete:
- Absolute zero: −273.15 °C / 0 K
- Freezing point of water: 0 °C / 273.15 K
- Room temperature (roughly 20–22 °C): about 293–295 K
- Human body temperature (37 °C): 310.15 K
- Boiling point of water: 100 °C / 373.15 K
You’ll notice that Kelvin values for everyday temperatures look large. That’s because you’re measuring from a starting point far below anything you encounter in daily life. A pleasant spring day of 20 °C is already 293 K.
Why Science Uses Kelvin
Most scientific equations that involve temperature require Kelvin. The ideal gas law, for example, relates the pressure, volume, and temperature of a gas, and it only works when temperature is expressed in Kelvin. Using Celsius in that equation would break the math because the scale includes negative numbers that don’t correspond to a true ratio of thermal energy. Doubling the Kelvin temperature of a gas (say, from 300 K to 600 K) genuinely doubles the average energy of its molecules. Doubling a Celsius reading from 30 °C to 60 °C does not.
This is the core reason Kelvin exists as a separate scale. It’s an absolute measure: 0 K means zero thermal energy, and every value above it represents a proportional amount of energy. Celsius is relative, anchored to water rather than to a fundamental physical boundary. For cooking, weather forecasts, and medicine, Celsius works perfectly well. For physics, chemistry, and engineering calculations, Kelvin is essential.
How the Kelvin Is Officially Defined
Until recently, the kelvin was defined using the triple point of water, the precise temperature and pressure at which water can exist simultaneously as ice, liquid, and vapor. In 2018, the international scientific community updated the definition to tie it to a fundamental constant of nature called the Boltzmann constant, which links the energy of individual particles to temperature. The fixed value is 1.380649 × 10⁻²³ joules per kelvin.
For everyday conversions, this change makes no practical difference. The relationship between Kelvin and Celsius stays exactly the same. But anchoring the kelvin to a universal constant rather than to the behavior of a specific substance makes the measurement system more precise at extreme temperatures, both very hot and very cold, where the old water-based definition was harder to apply.