Celsius to Kelvin

Kelvin is an absolute scale: 0 K is absolute zero, the point where a system has its minimum possible thermal energy. Laws such as the ideal gas law, PV = nRT, only work with absolute temperatures. Heating air from 20 °C to 40 °C does not double its pressure in a sealed tank, because in kelvin that is 293.15 K to 313.15 K, an increase of just under 7%.

The idea goes back to William Thomson, later Lord Kelvin, who proposed an absolute temperature scale in 1848. Chemistry, physics, cryogenics and astronomy all use it, and so does any formula that multiplies or divides by temperature.

From 1954 the scale was tied to the triple point of water, set at exactly 273.16 K, which is 0.01 °C. Since the SI revision of 2019, the kelvin has instead been defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K. The Celsius scale is defined from kelvin, so the offset of 273.15 is exact by definition.

A conference in 1967 also changed the name and symbol: it is the kelvin, written K, not degrees Kelvin or °K. You write 300 K, while Celsius keeps its degree sign, 26.85 °C.

Rounding the offset to 273 is fine for rough work but adds a 0.15 K error that shows up in lab reports and textbook answers. Another mistake is converting a temperature difference: a rise of 10 °C is a rise of exactly 10 K, with no 273.15 added, because the degree size is the same.

To go back, use kelvin to Celsius. Starting from Fahrenheit, Fahrenheit to kelvin handles the extra step.