We proposed an Rmax estimation method based on the radius of the 50 knot wind (R₅₀). The data obtained during the passage of strong typhoons by a meteorological station network in the Japanese archipelago enabled us to derive the following simple formula, R_max = 0.23 R₅₀. The proposed method is expected to increase the reliability of storm surge prediction and contribute to disaster risk management, particularly in the western North Pacific.