Diameter of motor/power pulley
Diameter of machine pulley
Distance between shaft centers
Estimate torque changes across the drive.
Enter parameters and calculate to see results.
How the Pulley & Belt Calculator Works
When two pulleys are connected by a belt, they transmit rotational power from a driver (usually a motor) to a driven machine. Because the pulleys are linked by the belt, the belt travels at the exact same linear speed over both pulleys (assuming no slip).
This physical link means that a smaller pulley will spin faster than a larger pulley to keep up with the belt. This relationship allows us to change speeds and torque purely by altering pulley diameters.
Pulley RPM Formula
The driven RPM is inversely proportional to the diameter ratio.
Example: A 1450 RPM motor with a 100mm drive pulley is connected to a 200mm driven pulley.
1450 × (100 ÷ 200) = 725 RPM.
Torque Change Explained
Speed and torque share an inverse relationship. If you reduce the speed via pulleys, you increase the torque (turning force) by the same ratio, minus slight efficiency losses from belt friction.
Belt Length Formula
Calculating the exact length of an open belt (un-crossed) requires accounting for the straight runs between the pulleys and the curved wrap around each pulley. The standard geometric formula is:
L = 2C + 1.5708(D + d) + (D - d)² / 4C
- LTotal pitch length of the belt
- CShaft centre-to-centre distance
- DDiameter of larger pulley
- dDiameter of smaller pulley
How to Choose Pulley Sizes When You Only Know the RPM
If you know the motor RPM and the RPM required by the pump, fan, compressor or machine, the first step is to calculate the speed ratio. The calculator then tests common pulley sizes and suggests practical pulley combinations that come closest to the target RPM.
Why Shaft Size Affects Pulley Selection
A pulley ratio calculates the speed, but shaft size affects whether a pulley is practical to fit. A large shaft needs enough pulley hub material for the bore, keyway and locking bush. This is why very small pulleys should not be selected for large industrial shafts, even when the RPM calculation is correct. Always confirm pulley bore, keyway, belt type and machine safe speed before buying parts.
Where This Calculator Is Used
- Air compressors and blowers
- HVAC fans and extraction systems
- Water pumps and hydraulics
- Conveyor belt drives
- Lathes, mills and drill presses
- Agricultural machinery
- Custom workshop generator builds
Common Mistakes
Measuring outside diameter: Calculations should technically use the Pitch Diameter (where the belt sits inside the groove), not the raw outside diameter of the pulley.
Ignoring belt slip: V-belts can slip 1-3% under heavy load, meaning actual driven RPM is slightly lower than theoretical.
Too short centres: If the pulleys are too close, the belt won't have enough "wrap angle" on the smaller pulley, causing slippage.