Free engineering utility
Gear ratio calculator
Estimate output speed and the basic torque multiplication created by a reduction gearbox. Use the result to narrow the search—not to certify a replacement.
This calculator illustrates basic ratio relationships. It does not account for service factor, startup loads, thermal limits, shaft loads, duty cycle, lubricant, mounting, motor characteristics, or manufacturer derating. Verify final selection against manufacturer engineering data.
Basic gearbox ratio formula
For a simple reduction ratio, output speed is input speed divided by the reduction ratio. A 1,750 RPM motor driving a 20:1 reducer produces an ideal output speed of 87.5 RPM.
Output RPM = Input RPM ÷ Reduction RatioWhat about output torque?
In an ideal lossless calculation, torque increases in proportion to the reduction ratio. Real gearboxes have losses, so the calculator also shows an efficiency-adjusted illustration when you supply an estimated efficiency percentage.
Ideal Output Torque = Input Torque × Reduction RatioWhy ratio alone is not enough to choose a gearbox
A replacement also has to fit the application mechanically and thermally. Relevant factors can include shaft dimensions, mounting style and position, service factor, duty cycle, peak loads, motor interface, permissible radial/axial load, environmental rating, lubricant and manufacturer-specific ratings.
Have the model number?
Search the verified catalog.
Use the manufacturer and model from the nameplate to find source-backed records and documented transitions.
Not sure what is on the nameplate? Use the identification guide →