Ice point (0 °C in kelvin)
| Value | 273.15 K |
| Status | Exact by definition — no uncertainty |
| Source | SI Brochure, 9th edition (2019) |
| Categories | ThermodynamicEngineering & Trade |
| nanokelvin | 273,150,000,000 nK |
| microkelvin | 273,150,000 μK |
| millikelvin | 273,150 mK |
| degree Fahrenheit | 32 °F |
| degree Rankine | 491.67 °R |
| kelvin | 273.15 K |
| degree Celsius | 0 °C |
| degree Réaumur | -2.8421709e-14 °Ré |
| kilokelvin | 0.27315 kK |
| megakelvin | 0.00027315 MK |
| gigakelvin | 2.7315000e-07 GK |
Learning zone
This is the offset in the definition of the Celsius scale, t/°C = T/K − 273.15, and it is exact by definition rather than measured. It is also the single most common source of arithmetic error in thermodynamics: gas laws, Carnot efficiency and radiation calculations all demand absolute temperature, and feeding them Celsius produces answers that look plausible and are badly wrong.
The number is not quite the freezing point of water. Air-saturated water at 1 atm freezes at 273.1500 K to within a fraction of a millikelvin by happy historical accident, but the ice point stopped being the definition in 1954, when the scale was anchored to the triple point instead. Note also the difference between a temperature and a temperature difference: a change of 1 °C equals a change of 1 K exactly, with no offset at all.