Pulley Ratio Calculator
Estimate speed reduction or increase in a two-pulley belt drive.
How to use this calculator
- Identify the pulley connected to the power source as the driver and the pulley receiving power as the driven pulley.
- Enter pitch or effective diameters when the manufacturer provides them. Outside diameter is easier to measure but can introduce error because the belt does not transmit motion exactly at the outer surface.
- Enter driver RPM and read the ideal driven RPM. A larger driven pulley produces a speed reduction; a smaller driven pulley produces a speed increase.
- Use the reported belt speed only as a kinematic value. Compare it with the belt and equipment manufacturer's published speed and power limits before selecting hardware.
- If the system uses multiple pulley stages, calculate each stage separately and multiply the stage ratios to obtain the total speed change.
Formula and assumptions
Driven RPM = driver RPM × driver effective diameter ÷ driven effective diameter. Reduction ratio = driven diameter ÷ driver diameter. Ideal belt speed is based on the circumference and RPM of the selected effective diameter.
GIIRR calculates idealized geometry and kinematics. Friction, slip, compliance, tire deformation, manufacturing tolerances and control-system behavior can move real measurements away from the theoretical result.
Worked example
A 3-inch effective driver pulley at 1,800 rpm driving a 9-inch effective pulley gives 1,800 × 3 ÷ 9 = 600 rpm, a 3:1 speed reduction. Replacing only the driven pulley with a 6-inch effective diameter raises ideal driven speed to 900 rpm. Real systems can differ slightly because of belt slip and the difference between outside and effective diameter.
Frequently asked questions
Do pulley outside diameters always work?
They are an approximation. Pitch or effective diameter better represents the line at which belt motion is transmitted and is the preferred value for accurate work.
Does belt thickness change the result?
It can change the effective rolling diameter. Manufacturer effective-diameter data is better than assuming the belt runs exactly on the pulley outside diameter.
Will a 2:1 pulley ratio always give exactly half the RPM?
Ideally yes, but real belt slip, elastic effects and measurement error can move actual driven RPM away from the theoretical value.
Can I use radius instead of diameter?
Yes if you use radius for both pulleys, because the common factor cancels. Do not mix a radius for one pulley with a diameter for the other.
Does this tell me which belt to buy?
No. Belt selection also depends on groove profile, center distance, wrap angle, power, speed, tensioning and manufacturer ratings.
For safety-critical, legal or fitment decisions, verify the result against manufacturer documentation and physical measurements.
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