Gear Ratio Calculator
Compare the tooth counts of a driver and driven gear and estimate ideal output speed for a simple gear pair.
How to use this calculator
- Enter the number of teeth on each gear and the driver input speed in RPM.
- Review the worked example for gear ratio calculator and compare the inputs with your own units.
- Change one assumption at a time and recalculate if you need a second scenario.
Simple gear pair
Driven-to-driver ratio = driven gear teeth ÷ driver gear teeth; ideal output RPM = input RPM ÷ ratio.
- The calculation is the magnitude ratio for a simple external gear pair and omits losses and compound arrangements.
- The result is calculated from the values entered; the page does not independently verify assumptions, data quality or real-world suitability.
Worked examples
A 12-tooth driver
A 12-tooth driver turns at 900 RPM and drives a gear with 36 teeth.
36 ÷ 12 = 3; 900 ÷ 3 = 300 RPM.
The ratio is 3:1 and ideal driven speed is 300 RPM.
Understanding the result
A larger driven gear turns more slowly while producing a higher ideal torque ratio.
Direction reverses for a simple external gear pair; efficiency, tooth geometry and load affect real behaviour.
Important notes
Check whether the application has additional stages, idlers, belt slip or chain sprockets.
Displayed figures are rounded for readability. Preserve more precision during intermediate calculations and verify the labels and units before using the estimate.
Frequently asked questions
What does the Gear Ratio Calculator show?
A larger driven gear turns more slowly while producing a higher ideal torque ratio.
What assumptions should I check?
Check whether the application has additional stages, idlers, belt slip or chain sprockets.
Does this replace expert or authoritative guidance?
Direction reverses for a simple external gear pair; efficiency, tooth geometry and load affect real behaviour.
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Learn the method behind the number or compare the terminology before using the calculator.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.