A model has been developed for the local configuration factor for the radiant interchange between the internal surface of the cone heater and the specimen's surface in the cone calorimeter. The incident heat flux under the cone was characterised in a series of experiments as a function of spatial coordinates. Both the emissivity and the temperature of the heating element were estimated from the experimental data. Significant variations in the surface temperature, and therefore in radiosity, were observed. The expression for the configuration factor was applied to estimate uncertainties in the measured variables due to tolerances in independent parameters. Plots were provided of the configuration factor and isolines of the radiant heat flux at various heights below the base of the cone. A comparison was carried out between predicted and measured data, demonstrating good agreement. The model predicted that, at the standard location of the specimen of 25 mm below the frustum, the radiant flux is uniform but only within the central area of the sample. Toward the edges and corners, the flux decreases by 2.6 and 15.9%, respectively, in comparison to that along the centreline.