A gearbox trades speed for torque at the same power. Divide the speed by the ratio, multiply the torque by the ratio, then take off the losses. Enter your numbers below.
Use the efficiency from the gearbox datasheet. Worm and cheap spur boxes can be far below planetary ones.
How it is calculated
Output speed = motor speed ÷ ratio. Output torque = motor torque × ratio × efficiency. Output power = torque × angular speed, where angular speed in rad/s = RPM × 2π ÷ 60. Power out is always lower than power in, by the losses.
A worked example
A small DC motor turns at 300 RPM with 0.05 N·m. A 30:1 gearbox at 70% efficiency gives 300 ÷ 30 = 10 RPM and 0.05 × 30 × 0.70 = 1.05 N·m. That is about 10.7 kg·cm at the output shaft. Power out is 1.05 × (10 × 2π ÷ 60) = 1.10 W.
Common mistakes
- Using no-load speed and stall torque together. They never happen at the same time; use the working point.
- Forgetting efficiency. A 30:1 box does not give a full 30 times the torque.
- Ignoring backlash and shaft strength, which matter more than the ratio in precise joints.
Related
servo torque calculator, robot gearhead selection, measuring backlash.
Last reviewed: November 2026. The calculator runs in your browser and sends nothing anywhere. Results are ideal-case figures; add margin before you order parts.