Spatial Assessment of External Heat Flux in Cone Calorimeter Measurements Using Area-Weighted Averaging

External heat flux is a primary thermal input parameter in cone calorimeter tests, with the nominal value conventionally established at the center of the specimen surface. However, the radiant heat flux generated by the cone heater specified in ISO 5660-1 is known to be spatially non-uniform. This study quantified the spatial distribution of external heat flux at nominal levels of 20, 35, and 50 kW/m2 and applied area-weighted spatial averaging to quantify the relationship between the nominal heat flux and the spatially averaged external heat flux over the specimen. The measured heat flux decreased radially toward the specimen boundaries, with local relative deviations reaching approximately −30% near the corners of a standard 0.100 × 0.100 m specimen. Despite this local non-uniformity, the normalized spatial distributions were similar across all three heat flux levels. For the entire specimen, the area-weighted relative deviations were −3.54%, −3.28%, and −3.41% at 20, 35, and 50 kW/m2, respectively, with comparable deviations in total external radiant power. These findings demonstrate that the magnitude of local heat-flux deviation should be distinguished from the specimen-wide spatially averaged deviation. The proposed area-weighted spatial interpretation provides a quantitative basis for relating the nominal external heat flux established at the specimen center to the spatially averaged external heat flux over the specimen under the ISO 5660-1 cone calorimeter configuration.