Thermal inertia is an extensive property of materials related to thermal conductivity and volumetric heat capacity. It is a term frequently used in modeling heat transfer mechanisms.

The term is, in reality, a scientific analogy, which makes use of a name borrowed from mechanics where "inertia" defines an effect that opposes the acceleration of a body. Similarly, thermal inertia is a measure of thermal mass and the thermal wave that controls the surface temperature of a material.

In heat transfer models, a higher value of the volumetric heat capacity means that the system will require a longer time to reach thermodynamic equilibrium.

The thermal inertia of a material is defined as the square root of the product between the thermal conductivity and the volumetric heat capacity, where the latter is the product between the density and the specific heat capacity.

The term is used, for example, when dynamic effects prevail in an abstract model, so that the steady-state calculation could lead to imprecise results, in which case it is said, for example: "this material has a high thermal inertia", or "thermal inertia plays an important role in this system".

In planetary science, the thermal inertia of surface materials is a fundamental property that controls the variations in diurnal, nocturnal and seasonal temperatures, where it varies depending on the geological materials that are close to the surface.

In remote sensing applications, thermal inertia represents a complex combination of particle size, rock abundance, bedrock outcrops, and degree of elevation. A rough approximation of thermal inertia can be obtained from the amplitude of the diurnal temperature curve (maximum temperature minus minimum temperature). The temperature of a material with low thermal inertia varies significantly throughout the day, while the temperature of a material with high thermal inertia does not change as drastically.

Understanding and deriving the thermal inertia mechanisms of a surface can help recognize small-scale features of the surface; in conjunction with other types of data, thermal inertia can help characterize surface materials and the geological processes responsible for the formation of these materials.

The thermal inertia of the oceans is a major influence on climate change: the degree of global warming that is ultimately predicted will depend on a fundamental change in radiative forcing, such as a sustained increase in greenhouse gas concentrations.