Before choosing a gearbox, please specify the gear ratio you need. After determining a ratio, multiply your servo motor rated torque by the ratio. The value should be in principle less than our gearbox rated output torque which can be found in the data sheets. Take other overloads which drive the motor into consideration to choose the max operating torque; this should not exceed 2 times of our gearbox rated torque. After determining the above, you can choose the smallest volume gearbox to save cost. You can send us the servo motor dimension drawing for model selection or advise the specific servo motor model number. If you have special requirements for precision, please clarify.