［Objective］ The current national standard GB 14907—2018 certifies the grade for intumescent fire-retardant coatings（IFCs） based solely on fire resistance time. Consequently，test reports for IFCs lack the critical parameter required for structural design—equivalent thermal resistance— making it difficult for structural engineers to directly determine the required coating thickness inpractice.［Methods］ In this paper，a novel engineering method utilizing the mandatory test itemsspecified in the national standard fire resistance test（namely，the maximum mid-span deflection of steel beams，the substrate peak temperature，and the fire resistance time）is proposed. Based on heat transfer theory and the high-temperature mechanical properties of steel，a computational model linking “national standard test parameters -equivalent thermal resistance -coating thickness”is established， Practical design curves are also provided by solving the transcendental equations.［Results］Validation based on actual test data from commercial IFC products demonstrated that the equivalent thermal resistance values acquired by this method deviated by no more than 12% from experimentally measuredvalues. Furthermore，the results were generally conservative，aligning with the principle of engineeringsafety design. This approach can eliminate the need for dedicated experiments to determine equivalentthermal resistance，significantly simplifying the process of fire-resistant design.［Conclusion］ The proposed method effectively bridges the critical data gap between the national product certification forfire-retardant coatings（GB 14907—2018） and the structural design code（GB 51249—2017）. It provides a scientific，direct，and operable solution for determining coating thickness in the engineering application of IFCs. This advancement is expected to promote the standardization use of IFCs instructural fire-resistant design.