In physics, thermal conductivity, Failed to parse (Missing texvc executable; please see math/README to configure.): k

), is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction.

Heat transfer across materials of high thermal conductivity occurs at a higher rate than across materials of low thermal conductivity. Correspondingly materials of high thermal conductivity are widely used in heat sink applications and materials of low thermal conductivity are used as thermal insulation. Thermal conductivity of materials is temperature dependent. The reciprocal of thermal conductivity is thermal resistivity.

In the International System of Units (SI), thermal conductivity is measured in watts per meter kelvin (W/(m·K)) or W·m⁻¹·K⁻¹.

In the imperial system of measurement thermal conductivity is measured in Btu/(hr·ft⋅F) where 1 Btu/(hr·ft⋅F) = 1.730735 W/(m·K).

Other units which are closely related to the thermal conductivity are in common use in the construction and textile industries. The construction industry makes use of units such as the R-value and the U-Value (thermal transmittance). Although related to the thermal conductivity of a material material used in an insulation product, R and U-values are dependent on the thickness of the product.