What is the value of the triple point of water in Kelvin?

The triple point of water has a temperature of 273.16 K. At this precisely defined combination of temperature and pressure, ordinary ice, liquid water, and water vapor coexist in equilibrium. The pressure is about 611.657 pascals, far below standard atmospheric pressure.

The triple point became central to thermometry because it provided a reproducible reference for the kelvin. From 1954 until the SI revision in 2019, the kelvin was defined using the triple-point temperature of specially specified water. The modern SI instead defines the kelvin through the Boltzmann constant, so the triple point is now experimentally measured rather than exact by definition.

A common mix-up is 273.15 K, which corresponds to 0 °C on the Celsius-to-kelvin conversion scale. The triple point is 0.01 °C, or 273.16 K. Also, water has other triple points involving high-pressure ice phases; this answer refers to the familiar ice-liquid-vapor point.