# Gear train calculator: output speed, torque and ratio

> Free gear train calculator for robots: enter motor speed, torque, gear ratio and efficiency to get output RPM and torque, with a worked example.

Canonical: https://roboticsengineering.in/gear-train-calculator/ | Published: 2026-11-03 | Updated: 2026-11-03

**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.

Motor speed (RPM)Motor torque (N·m)Gear ratio (N : 1)Efficiency (%)

Output speed: **–** RPMOutput torque: **–** N·m (– kg·cm)Output power: **–** W

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](https://roboticsengineering.in/servo-torque-calculator/), [robot gearhead selection](https://roboticsengineering.in/robot-gearhead-selection-india/), [measuring backlash](https://roboticsengineering.in/how-to-measure-gearbox-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.