Belts & Pulleys
Pulley Ratio Calculator
Enter driver and driven pulley diameters to calculate the ideal speed ratio for a no-slip belt drive.
Inputs
Formula
Pulley speed ratio
Worked example
Using the default values shown in the calculator, the same formula gives the following result. This is a quick sanity check for the calculation, not a design recommendation.
Inputs
- Driver pulley diameter
- 100 mm
- Driven pulley diameter
- 250 mm
Result
- Speed reduction ratio
- 2.5 :1
Related calculators
Continue your workflow with a related calculation.
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Calculate the driven pulley diameter needed for a target RPM.
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Calculate belt linear speed from pulley diameter and rotational speed.
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Belt Length Calculator
Calculate approximate open-belt length from pulley diameters and center distance, with metric and imperial results.
Frequently asked questions
What does a 2.5:1 pulley ratio mean?
Ideally, the driven shaft turns once for every 2.5 turns of the driver, ignoring belt slip.
Do the pulley diameters need the same unit?
No. Mixed supported units are normalized before the ratio is calculated.