273

Gases expand by very close to 1/273 of their volume at 0 °C for each additional degree Celsius, a regularity established by Charles and by Gay-Lussac around 1800. Running the relation backwards implies a temperature at which the volume would reach zero — the value now fixed at exactly −273.15 °C — and it was this extrapolation — not any measurement — that first located absolute zero. Kelvin proposed the thermodynamic scale on that basis in 1848. No gas actually behaves that way near the limit; every real one liquefies first.

The Celsius–Kelvin offset is exactly 273.15 by definition rather than by measurement, because the 2019 SI redefinition fixed the Boltzmann constant and then defined the degree Celsius as the kelvin with that offset applied. Before 2019 the scales were pinned to the triple point of water at exactly 273.16 K, which made 0 °C exactly 273.15 K by the same construction. The figure has been exact under both definitions; what changed is what it is exact relative to.

There are 273 point one five kelvin, the temperature of the ice point, which is zero degrees Celsius (worldwide, SI).

The Celsius and Kelvin scales use degrees of identical size but different zero points, offset by exactly 273.15: water freezes at 0 °C, which is 273.15 K. Converting between them is therefore addition alone, with no scaling.

Absolute zero — the lower limit of the thermodynamic temperature scale — is exactly −273.15 °C, or 0 kelvin. 273 is the rounded figure used in everyday references; the .15 is exact by definition, not a measurement uncertainty.