The determination of the required electrode length in the grounding system is critical for the design of lightning transient protection. The derivation of a generic expression to calculate the useful length of the grounding electrode is tedious due to the nonlinear phenomena. A novel technique is proposed to arrive at a generalized expression to calculate the useful length of the grounding electrode from the nonlinear correlations. The correlations between the useful length of the grounding electrode, soil resistivity, and lightning current characteristics are established from the simulation data using a transient model published recently. The proposed approach comprises of practical curve-fitting and optimization techniques. Using the proposed technique, a general expression is derived from the correlation between the electrode length with soil resistivity, lightning current rise time, and magnitude. The derived expression is compared with simulation results and conventional method to validate the proposed technique, accuracy, and applicability. The arrived expression can be utilized to compute the required length of the grounding electrode for any soil property, lightning, and power transient conditions.