
The SI base unit kelvin (K) is defined as follows:
The degree Celsius (°C) is the special name for the kelvin used to express Celsius temperatures. The degree Celsius and the kelvin are equal in size, so that the numerical value of a temperature difference or temperature interval is the same when expressed in either degrees Celsius or in kelvins.
See also: International Temperature Scales
SI base unit (kelvin)


The definition of the unit of thermodynamic temperature was given in substance by the 10th CGPM (1954, Resolution 3) which selected the triple point of water as the fundamental fixed point and assigned to it the temperature 273.16 K, so defining the unit. The 13th CGPM (1967/68, Resolution 3) adopted the name kelvin, symbol K, instead of "degree Kelvin", symbol °K, and defined the unit of thermodynamic temperature as follows (1967/68, Resolution 4):
The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
It follows that the thermodynamic temperature of the triple point of water is exactly 273.16 kelvins, T_{tpw} = 273.16 K.


The symbol, T_{tpw}, is used to denote the thermodynamic temperature of the triple point of water.


At its 2005 meeting the CIPM affirmed that:
This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: 0.000 155 76 mole of ^{2}H per mole of ^{1}H, 0.000 379 9 mole of ^{17}O per mole of ^{16}O, and 0.002 005 2 mole of ^{18}O per mole of ^{16}O.
Because of the manner in which temperature scales used to be defined, it remains common practice to express a thermodynamic temperature, symbol T, in terms of its difference from the reference temperature T_{0} = 273.15 K, the ice point. This difference is called the Celsius temperature, symbol t, which is defined by the quantity equation:
t = T – T_{0}.
The unit of Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the kelvin. A difference or interval of temperature may be expressed in kelvins or in degrees Celsius (13th CGPM, 1967/68, Resolution 3, mentioned above), the numerical value of the temperature difference being the same. However, the numerical value of a Celsius temperature expressed in degrees Celsius is related to the numerical value of the thermodynamic temperature expressed in kelvins by the relation
t/°C = T/K – 273.15.
The kelvin and the degree Celsius are also units of the International Temperature Scale of 1990 (ITS90) adopted by the CIPM in 1989 in its Recommendation 5 (CI1989).



Decisions relating to the base units of the SI




