How to Use the Gear Ratio Calculator
When a motor and a machine are connected through gears, the tooth-count ratio between the drive and driven gears determines both output speed and the torque delivered. Whether you're designing a gearbox or replacing gears on existing equipment, it helps to know in advance which tooth combination gets you the output speed you need.
This calculator uses the basic gear train relationships: gear ratio = driven gear teeth ÷ drive gear teeth, and output RPM = input RPM × (drive gear teeth ÷ driven gear teeth). The gear with more teeth turns slower and, in theory, can transmit proportionally more torque — a fundamental relationship in gear power transmission.
The torque multiplier shown is a theoretical value assuming 100% gear efficiency. Real gear trains typically run at 95-98% efficiency due to tooth friction and bearing losses, so actual delivered torque is somewhat lower. When sizing a gearbox for multi-stage reduction, multiply the ratio of each stage to get the overall reduction, and multiply each stage's efficiency for the combined efficiency — use manufacturer efficiency data for precise designs.
Frequently Asked Questions
Gear ratio = driven gear teeth ÷ drive gear teeth. For example, a 20-tooth drive gear and a 60-tooth driven gear gives a 3:1 ratio, and output speed becomes one-third of input speed.
Under ideal conditions (100% efficiency, no losses), torque increases by the same ratio that speed decreases. In practice, gear efficiency (typically 95-98%) introduces some loss, so actual torque is slightly below the theoretical value.