In thermodynamics, thermal pressure (also known as the thermal pressure coefficient) is a measure of the relative pressure change of a fluid or a solid as a response to a temperature change at constant volume. The concept is related to the Pressure-Temperature Law, also known as Amontonss law or Gay-Lussacs law.
In general pressure, () can be written as the following sum: .
 is the pressure required to compress the material from its volume  to volume  at a constant temperature . The second term expresses the change in thermal pressure  . This is the pressure change at constant volume due to the temperature difference between  and . Thus, it is the pressure change along an isochore of the material.
The thermal pressure  is customarily expressed in its simple form as