The transfer of heat energy from one place to another per unit time per unit cross-sectional area over which heat transfer takes place. Heat transfer calculations are based on the area of heating surface. In the case of tubes or pipes, heat fluxes may be based with reference to either the inside or the outside areas. While the choice is arbitrary, it must be stated as the magnitude of the heat flux is not the same for both. The SI units are W m⁻².