Celsius to Kelvin
Convert celsius (°C) to kelvin (K) instantly. For weather, cooking and science alike.
Celsius → Kelvin
274.15
K
1 °C = 274.15 K
Frequently asked questions
Add 273.15. The two scales use identical degree sizes and differ only in where zero sits, so a change of one degree Celsius is a change of exactly one kelvin. Kelvin takes no degree symbol — it is written 300 K, not 300°K.
Because Kelvin has a true zero at absolute zero, which makes its values meaningfully multiplicable — 400 K really is twice as hot as 200 K thermodynamically, whereas 40°C is not twice 20°C. The gas laws, blackbody radiation and thermal efficiency calculations all require absolute temperature, and using Celsius in them gives a wrong answer.
It describes the colour of a light source by reference to the temperature of a blackbody radiator producing similar light. 2,700 K is warm white like an incandescent bulb, 4,000 K is neutral, and 6,500 K is cool daylight. Counter-intuitively, higher kelvin values look cooler to the eye, so the physics and the aesthetic vocabulary run in opposite directions.
Add 273.15
Converting Celsius to Kelvin is pure addition, because the two scales use identical degree sizes and differ only in where zero is placed.
K = °C + 273.15
A change of one degree Celsius is a change of exactly one kelvin. Only the offset differs.
Absolute zero
Kelvin's zero is absolute zero — the temperature at which all classical thermal motion ceases and a system reaches its minimum possible energy. It is −273.15°C, and it is a genuine physical floor rather than an arbitrary reference point.
That is what makes Kelvin the SI base unit of temperature and the scale used throughout physics and chemistry. Because it has a true zero, kelvin values can be meaningfully multiplied and divided — 400 K really is twice as hot as 200 K in a thermodynamic sense, whereas 40°C is not twice 20°C.
This property matters in practice: the gas laws, blackbody radiation, thermal efficiency calculations and the Stefan-Boltzmann law all require absolute temperature. Using Celsius in any of them gives a wrong answer, often dramatically so, and it is a standard early mistake in physics.
Notation
Kelvin takes no degree symbol. It is written 300 K, not 300°K, and the unit is "kelvin" in lower case when spelled out even though the symbol is capitalised after its namesake, Lord Kelvin.
The reference points
- 0 K = −273.15°C — absolute zero
- 77 K = −196.15°C — liquid nitrogen boils
- 273.15 K = 0°C — water freezes
- 293.15 K = 20°C — room temperature
- 298.15 K = 25°C — the standard reference temperature in chemistry
- 310.15 K = 37°C — body temperature
- 373.15 K = 100°C — water boils
- 5,778 K — the surface of the Sun
Where Kelvin appears outside physics
Colour temperature in lighting and photography, which is the most common everyday encounter. A light source is described by the temperature of the blackbody radiator that would produce a similar colour:
- 2,700 K — warm white, the colour of an incandescent bulb
- 3,000 K — soft white
- 4,000 K — neutral white
- 5,000–5,500 K — daylight
- 6,500 K — cool daylight, the standard reference white for displays
Counter-intuitively, higher colour temperature looks "cooler" to the eye — bluer light has a higher kelvin value while being described as cool, and warm orange light has a lower one. The physics and the aesthetic vocabulary run in opposite directions.
Astronomy, where stellar surface temperatures are given in kelvin.
Cryogenics and superconductivity, where the interesting temperatures are close to zero and only an absolute scale is convenient.
Rankine, for completeness
An absolute scale using Fahrenheit-sized degrees also exists — the Rankine scale, where absolute zero is 0 °R and water freezes at 491.67 °R. It appears occasionally in American engineering thermodynamics and essentially nowhere else.