This paper presents the results of an optimization problem to determine the optimal gear ratios of a four-stage helical gearbox to achieve the minimum gearbox bottom area. To do that, a simulation experiment was conducted. In addition, the influence of six main design parameters including the surface width coefficient of the first and second stages, the allowable contact stresses of the first and second stages, and the output torque have been evaluated. Moreover, regression equations to determine the optimal gear ratios of the gearbox have been proposed.