Gear Ratio Calculator
Compare the tooth counts of driving and driven gears to see how torque and speed change in your drivetrain.
Verify gear compatibility before installation.
Examples
- Driver 12 teeth, driven 36 teeth ⇒ 3:1 ratio (torque triples)
- Driver 18 teeth, driven 27 teeth ⇒ 1.5:1 ratio (speed drops 33%)
- Driver 20 teeth, driven 20 teeth ⇒ 1:1 ratio (no change)
FAQ
What is gear ratio?
It expresses how many turns the input gear makes relative to the output gear, affecting torque and speed.
How do I convert the decimal ratio to a colon format?
Express the decimal as driven:driving. For example, 2.5 becomes 2.5:1, meaning the driven gear turns once for every 2.5 input turns.
Can I calculate speed change from the ratio?
Yes. Divide the input RPM by the ratio to estimate output RPM (ignoring losses).
Does gear direction matter?
Direction does not affect the ratio, but it determines whether the driven gear spins clockwise or counterclockwise.
Additional Information
- Gear ratio equals driven teeth ÷ driving teeth. Ratios above 1 increase torque while reducing speed.
- Use whole numbers for teeth counts; fractional teeth are not physically possible on standard gears.
- For multi-stage gearboxes, multiply the ratios of each stage to get the overall ratio.
- Lubrication, backlash, and efficiency losses are not included—this is an ideal mechanical ratio.