This paper introduces a new methodology for computing the volume of two-stage helical gearboxes with the first double gear sets. The volume of the gearbox is mentioned as the optimization problem. The six design factors such as the total gearbox ratio, the coefficient of the face width (CFW) of the first gear stage, coefficient of wheel face width (CWFW) of the second gear stage, the allowable contact stress (ACS) of the first stage, the ACS of the second stage, and the output torque were considered to design a screening experiment. To explore the influences of the design parameters on the optimum gear ratio, Minitab @18 was used to set up experiments. Moreover, technique tools i.e., regression and variance analysis were utilized to build the explicit model and to evaluate the reasonability of the model with experimental data. The results show that the optimal gear ratio can be simply computed from given design factors.