What is Kelvin temperature?
The kelvin (symbol K) is the SI base unit of temperature. Where Celsius and Fahrenheit were built around water and the human body, the Kelvin scale is built around absolute zero: the temperature at which thermal motion is at its absolute minimum. That point is 0 K, which is −273.15 °C or −459.67 °F. Because the scale starts at the coldest anything can be, kelvin values are never negative.
The scale is named after William Thomson, Lord Kelvin, who proposed an absolute temperature scale in 1848. A kelvin is the same size as a Celsius degree, so converting between the two is a matter of adding or subtracting 273.15. Since 2019 the kelvin has been defined by fixing the value of the Boltzmann constant, which ties temperature directly to the energy of particles rather than to any particular substance.
Kelvin is the working unit of physics, chemistry, astronomy and engineering, and it shows up in daily life more than you might think: the colour temperature of a light bulb (a "warm" 2700 K bulb versus a "cool" 6500 K one) is a kelvin figure.
Conversion formulas
| From | To kelvin | Absolute zero | Water freezes | Water boils |
|---|---|---|---|---|
| Celsius (°C) | K = °C + 273.15 | −273.15 °C | 0 °C | 100 °C |
| Fahrenheit (°F) | K = (°F + 459.67) × 5⁄9 | −459.67 °F | 32 °F | 212 °F |
| Rankine (°R) | K = °R × 5⁄9 | 0 °R | 491.67 °R | 671.67 °R |
| Réaumur (°Ré) | K = °Ré × 1.25 + 273.15 | −218.52 °Ré | 0 °Ré | 80 °Ré |
Going the other way: °C = K − 273.15, °F = K × 9⁄5 − 459.67, °R = K × 9⁄5, °Ré = (K − 273.15) × 0.8. The converter above shows all four alongside the kelvin result.
Common temperatures in kelvin
| Reference | Kelvin | Celsius | Fahrenheit |
|---|---|---|---|
| Absolute zero | 0 K | −273.15 °C | −459.67 °F |
| Liquid helium boils | 4.22 K | −268.93 °C | −452.07 °F |
| Liquid nitrogen boils | 77.36 K | −195.79 °C | −320.42 °F |
| Dry ice sublimates | 194.65 K | −78.5 °C | −109.3 °F |
| Water freezes | 273.15 K | 0 °C | 32 °F |
| Room temperature | 293.15 K | 20 °C | 68 °F |
| Human body | 310.15 K | 37 °C | 98.6 °F |
| Water boils (sea level) | 373.15 K | 100 °C | 212 °F |
| Surface of the Sun | 5,772 K | 5,498.85 °C | 9,929.93 °F |
Frequently asked questions
How do you convert Celsius to kelvin?
Add 273.15. K = °C + 273.15. For example, 25 °C is 298.15 K. Kelvin and Celsius use the same size of degree; the scales are just offset from each other.
How do you convert Fahrenheit to kelvin?
Add 459.67, then multiply by 5⁄9. K = (°F + 459.67) × 5⁄9. For example, 98.6 °F is 310.15 K.
Why is it "kelvin" and not "degrees Kelvin"?
Because kelvin is an absolute unit, not a point on a relative scale. In 1967 the General Conference on Weights and Measures renamed the unit from "degree Kelvin" (°K) to "kelvin" (K). You write 300 K, never 300 °K or "300 degrees Kelvin". The unit name is lowercase; the scale and the man are capitalised.
Can a temperature in kelvin be negative?
No. 0 K is absolute zero, the lowest possible temperature. Any Celsius value below −273.15 °C or Fahrenheit value below −459.67 °F is physically impossible, and this converter flags it instead of printing a negative kelvin figure.
What is 0 K in Celsius and Fahrenheit?
0 K is −273.15 °C and −459.67 °F. It is also 0 °R on the Rankine scale, which is an absolute scale with Fahrenheit-sized degrees.
Want the history, the 2019 redefinition, and where kelvin is actually used?
Read about the Kelvin scale