Heat flux

In physics and engineering, heat flux or thermal flux, sometimes also referred to as heat flux density, heat-flow density or heat-flow rate intensity, is a flow of energy per unit area per unit time. Its SI units are watts per square metre (W/m²). It has both a direction and a magnitude, and so it is a vector quantity. To define the heat flux at a certain point in space, one takes the limiting case where the size of the surface becomes infinitesimally small.

Heat flux
Heat flux ${\displaystyle {\vec {\phi }}_{\mathrm {q} }}$ through a surface.
Common symbols	${\displaystyle {\vec {\phi }}_{\mathrm {q} }}$
SI unit	W/m2
Other units	Btu/(h⋅ft2)
In SI base units	kg⋅s−3
Dimension	${\displaystyle {\mathsf {M}}{\mathsf {T}}^{-3}}$

Heat flux is often denoted ${\displaystyle {\vec {\phi }}_{\mathrm {q} }}$, the subscript q specifying heat flux, as opposed to mass or momentum flux. Fourier's law is an important application of these concepts.