Pulley Ratio Converter
Calculate driver and driven pulley RPM and diameter ratios.
Solve for any unknown pulley variable.
Enter any three values — the fourth calculates automatically.
Pulley Ratio
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Pulley systems transfer rotational motion between shafts using a belt. The relationship between pulley diameters and their speeds (RPM) follows a simple inverse ratio.
The fundamental pulley formula:
- Driver RPM x Driver Diameter = Driven RPM x Driven Diameter
- Or equivalently: RPM1 x D1 = RPM2 x D2
Solving for each variable:
- Driven RPM = (Driver RPM x Driver Diameter) / Driven Diameter
- Driver RPM = (Driven RPM x Driven Diameter) / Driver Diameter
- Driven Diameter = (Driver RPM x Driver Diameter) / Driven RPM
- Driver Diameter = (Driven RPM x Driven Diameter) / Driver RPM
The speed ratio (also called transmission ratio):
- Speed Ratio = Driver Diameter / Driven Diameter = Driven RPM / Driver RPM
- A ratio greater than 1 means speed increase (overdrive).
- A ratio less than 1 means speed reduction (underdrive).
- A ratio of exactly 1 means both pulleys spin at the same speed.
Practical example: A motor runs at 1,750 RPM with a 3-inch pulley driving a 9-inch pulley on a machine.
- Driven RPM = (1,750 x 3) / 9 = 583 RPM
- Speed ratio = 3/9 = 1:3 (the machine runs at one-third the motor speed)
Common applications:
| Application | Typical Speed Ratio | Purpose |
|---|---|---|
| Drill press | 1:1 to 1:5 | Variable speed for different materials |
| Lathe | 1:2 to 1:6 | Reduce motor speed for heavy cutting |
| Air compressor | 1:2 to 1:4 | Reduce speed to match compressor rating |
| Generator | 2:1 to 3:1 | Increase speed to reach generator RPM |
| Table saw | 1:1 to 1:1.5 | Slight speed reduction for blade RPM |
Important design considerations:
- The belt speed (feet per minute) should not exceed the manufacturer’s recommendation, typically 5,000-6,500 FPM for standard V-belts.
- Belt speed = pi x Pulley Diameter (ft) x RPM.
- Very high speed ratios (above 6:1) should use multiple stages rather than a single belt drive.
- Torque is inversely proportional to speed: when you reduce speed by half, you double the available torque.
- When selecting pulleys, consider the minimum recommended pulley diameter for the belt width being used.