# ISO 31-4

ISO 31-4 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to heat. It is superseded by ISO 80000-5.

Its definitions include:

Quantity Unit Remarks
NameSymbolNameSymbolDefinition
thermodynamic temperatureT, (Θ)kelvinKThe kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.For practical measurements, the International Temperature Scale of 1990 defines several fixed points and interpolation procedures.
Celsius temperaturet, φdegree Celsius°CThe degree Celsius is a special name for the unit kelvin, for use in stating values of Celsius temperature.t = TT0, where T0 = 273.15 K
linear expansion coefficientαlreciprocal kelvin, kelvin to the power negative 1K−1${\displaystyle \alpha _{l}={\frac {1}{l}}{\frac {l}{T}}}$
...

Annex A of ISO 31-4 lists units of heat based on the foot, pound and second and some other units, including the degree Rankine, degree Fahrenheit, British thermal unit and others. Annex B lists conversion factors for three versions of the calorie.