Principles of heat flux measurement by circular foil heat flux sensors, also known as Gardon gages, are analyzed in a mixed convection/radiation environment, such as that occuring in the measurement of heat feedback from flames to the solid surface in vertical wall fires. A mathematical model is employed to derive the correction needed to determine the true heat flux from the flux indicated by the gage output using a calibration which is obtained under a radiative heat flux. It is shown that the correction is strongly dependent on the gage size and the heat transfer coefficient of thermal convection. Also, the effect of using a water-cooled gage for a surface at a temperature different from the gage base is studied. A method is proposed for estimating these corrections using charts.