Pulley Ratio Calculator
Belt Drives Follow the Same Logic as Gears, With One Less Part
A belt-and-pulley system trades gear teeth for pulley diameter, but the underlying relationship is the same: the ratio between the two pulley sizes sets how much the output speed drops or rises relative to the input. No meshing teeth means less precision than a gear train, but also simpler, quieter, and cheaper power transmission for many applications.
The Formula
Pulley Ratio = Driven Diameter / Driver Diameter
Driven RPM = Driver RPM / Pulley Ratio
Driven RPM = Driver RPM / Pulley Ratio
Worked Examples
| Driver Diameter | Driven Diameter | Driver Speed | Ratio | Driven Speed |
|---|---|---|---|---|
| 100 mm | 300 mm | 1,750 rpm | 3.0:1 | 583.33 rpm |
| 150 mm | 50 mm | 900 rpm | 0.333:1 | 2,700.27 rpm |
A driven pulley smaller than the driver always speeds up the output, exactly as a driven pulley larger than the driver always slows it down — the second example above is a step-up drive, not a step-down.
Where This Calculation Matters
- Machine drive design — selecting pulley sizes to bring a motor's output speed down (or up) to what a driven component like a fan, pump, or spindle needs.
- Retrofit speed changes — calculating what pulley swap is needed to change an existing machine's operating speed without replacing the motor.
- Belt and pulley system troubleshooting — confirming whether an observed output speed matches what the installed pulley sizes should produce.
How to Use This Calculator
- Enter Driver Pulley Diameter (mm) and Driven Pulley Diameter (mm).
- Enter Driver Speed (RPM).
- Select Calculate to get the pulley ratio and the resulting driven speed.
Related Calculations
For the equivalent calculation using gears instead of belts, see the Gear Design Calculator.