Pulley Speed Ratio Calculator
Find the driven pulley speed and belt drive ratio from two pulley diameters and the driver RPM.
How it works
Driven rpm = driver rpm × (driver diameter ÷ driven diameter) · Drive ratio = driven ÷ driver diameter
A belt moves at the same linear speed around both pulleys, so a smaller pulley must spin faster to keep up and a larger one turns slower. The driven speed is therefore the driver speed scaled by the diameter ratio — the smaller the output pulley relative to the input, the faster it runs. As with gears, gaining speed costs torque and vice versa.
Worked example
A 4-inch driver pulley on a 1,725 rpm motor connected to an 8-inch driven pulley gives 1,725 × (4 ÷ 8) = 862.5 rpm at the output. That is a 2:1 reduction, so the output turns half as fast with roughly double the torque.
Sources: Belt (mechanical) — Wikipedia
Frequently asked questions
Should I use pitch or outside diameter?
Use the pitch diameter where the belt actually rides for the most accurate speed. Outside diameter is close for flat belts and thin V-belts, but for precise ratios the pitch diameter is the correct figure.
Does belt thickness change the ratio?
Only slightly. The ratio depends on the effective (pitch) diameters. A thick belt raises the effective diameter of both pulleys a little, which largely cancels out in the ratio.
How is this different from a gear ratio?
The idea is identical — output speed scales with the size ratio — but pulleys use diameters instead of tooth counts, and a belt lets the two shafts sit farther apart and can slip under overload.
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This calculator is for educational purposes. Double-check important results before acting on them.