Pulley Ratio Converter
Calculate pulley RPM, diameter, belt speed, and mechanical advantage ratio.
Solve for any unknown in a two-pulley belt drive for motors and machinery.
Enter any three values — the fourth calculates automatically.
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.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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