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 It is superseded by ISO 80000-5.^[1]

Its definitions include:

Quantity		Unit			Remarks
Name	Symbol	Name	Symbol	Definition
thermodynamic temperature	T, (Θ)	kelvin	K	The 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 temperature	t, φ	degree Celsius	°C	The degree Celsius is a special name for the unit kelvin, for use in stating values of Celsius temperature.	t = T − T0, where T0 = 273.15 K
linear expansion coefficient	αl	reciprocal kelvin, kelvin to the power negative 1	K^−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.

References

1. "ISO 31-4:1992 Quantities and units". International Organization for Standardization. 1992. Retrieved 14 June 2025.