Pulley Calculator
Calculate driven pulley speed and belt speed for a two-pulley belt drive from pulley diameters and driver RPM.
Enter your values
Driver and driven pulley diameters, and driver speed.
How this calculator works
For a two-pulley belt drive with no slip, belt surface speed is the same at both pulleys, which gives the driven pulley speed:
N₁, D₁ are driver speed and diameter; N₂, D₂ are driven speed and diameter. Belt linear speed is:
with D in meters and N in RPM, giving V in m/s. This should be the same whether calculated from the driver or driven pulley, since the belt speed is common to both.
Reading the result
| D₂ / D₁ | Effect |
|---|---|
| > 1 (larger driven pulley) | Speed reduction |
| < 1 (smaller driven pulley) | Speed increase |
Frequently asked questions
Does this account for belt slip?
No — this is the ideal, no-slip case. Real V-belt and flat-belt drives typically run with 1–3% slip under load, so actual driven speed will be slightly lower than calculated.
What's a reasonable belt speed?
Most V-belt drives are designed for belt speeds in the range of 5–30 m/s — very low speeds waste capacity, and very high speeds increase centrifugal losses and belt wear.