In physics, thermal conductivity, k, is the property of a material describing its ability to conduct heat. It appears primarily in Fourier's Law for heat conduction. Thermal conductivity is measured in watts per kelvin-meter (W·K−1·m−1, i.e. W/(K·m).

There is also a measure known as heat transfer coefficient: the quantity of heat that passes in unit time through unit area of a plate of particular thickness when its opposite faces differ in temperature by one kelvin.

heat transfer coefficient = k/L, measured in W·K−1·m-2

When thermal resistances occur in series, they are additive. So when heat flows through two components each with a resistance of 1 °C/W, the total resistance is 2 °C/W.

While it might seem similar to heat resistance, a fabric's thermal resistance refers to the fabric's thickness. Even fabrics with a high thermal conductivity like cotton may have a higher thermal resistance if that cotton is put in many layers to increase its thickness. So a fabric's thermal resistance is how long it actually takes a heat transfer to occur as heat moves through the fabric in one direction at the speed given by that fabric's heat conductivity.

The R-value is a measure of thermal resistance used in the building and construction industry.