The efficiency of a railgun can be defined as the exchanged transferred electrical energy to the kinetic energy of the armature. According to the good relation between the applied force to the armature and the inductance gradient (L'), the importance of this parameter calculation is obviously clear. There are several researchers who calculated L' using different numerical and analytical methods in these years. The analytical method to solve the simple problems is suitable, and numerical methods include sufficient coding time and programmer performance time. A method will be introduced in this paper to extract a formula or formulas of L' using structural railgun parameters which can cause the results similar to the results of the numerical methods. This method has not any time problem, and so, there is a good accuracy of data analysis. In this technique, which is called intelligent estimation method, according to the behavior of the railgun magnetism and its geometrical structure, an equation with unknown multiplications for L' formula is defined. Then, by applying the reported data in different studies for L', these equations can be found. Finally, the proposed method introduces a closed formula for a given structure of railgun. A rectangular configuration is investigated as a case study in this paper.