Temperature scales are defined by the numerical value assigned to a standard fixed point. By international agreement the standard fixed point is the easily reproducible triple point of water: the state of equilibrium between steam, ice, and liquid water.

As a matter of convenience, the temperature at this standard fixed point is defined as 273.16 kelvins, abbreviated as 273.16 K.

This makes the temperature interval from the ice point1 (273.15 K) to the steam point2 equal to 100 K and thus in agreement over the interval with the Celsius scale discussed next, which assigns 100 Celsius degrees to it.

The kelvin is the SI base unit for temperature. The Celsius temperature scale (formerly called the centigrade scale) uses the unit degree Celsius ( C), which has the same magnitude as the kelvin.

Thus, temperature differences are identical on both scales. However, the zero point on the Celsius scale is shifted to 273.15 K, as shown by the following relationship between the Celsius temperature and the Kelvin temperature

T(°C) = T(K) - 273.15

From this it can be seen that on the Celsius scale the triple point of water is 0.01 C and that  0 K corresponds to 273.15 C.

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