Pulley RPM Calculator
Enter both pulley diameters and the driver speed to get the driven RPM and belt surface speed — with the V-belt speed limits flagged.
Open calculatorLast updated: 2026-09-15
How the calculation works
- Belt speed is common to both pulleys, so speed inversely tracks diameter: the smaller pulley spins faster.
- Pitch diameters (not outer diameters) set the true speed ratio for V-belts.
- Belt surface speed is checked against the practical V-belt ceiling of 25–30 m/s.
Worked example
Interpreting the result
This is the sizing tool for machine spindles, fans and pumps: pick pulley sizes to land the driven speed where the load wants it, then check belt speed and minimum pulley diameter for the belt section. Small pulleys below the belt's minimum diameter bend the cords excessively and fail early.
Assumptions
- No belt slip or creep (V-belts slip 1–2% in practice).
- Both pulleys in the same plane, open drive.
Limitations
- Timing belts use tooth counts, not diameters (see the gear ratio calculator for the same math).
- Does not size the belt section or power capacity.
Driven speed (RPM) with a 1,750 RPM motor
| Driver pulley | 100 mm driven | 150 mm driven | 200 mm driven | 250 mm driven |
|---|---|---|---|---|
| 60 mm | 1,050 | 700 | 525 | 420 |
| 80 mm | 1,400 | 933 | 700 | 560 |
| 100 mm | 1,750 | 1,167 | 875 | 700 |
Driven RPM = driver RPM × driver diameter ÷ driven diameter.
Worked scenario: an 80 mm pulley driving a 200 mm pulley
The ratio is 2.5 : 1, so a 1,750 RPM motor turns the driven shaft at 700 RPM, with the belt travelling at 7.33 m/s.
Choosing pulleys
- Pick the target speed first, then the diameters. A larger driven pulley slows the shaft.
- Keep pulleys above the minimum diameter for the belt section, or the belt flexes too sharply and wears quickly.
- Remember that belts slip a little, so measured speed is slightly lower than calculated.
Frequently asked questions
How do I slow down a motor with pulleys?
Put the small pulley on the motor and a larger one on the load: a 2:1 diameter ratio quarters nothing — it halves speed. Two-stage reductions multiply.
Why is my driven speed slightly off?
V-belt creep (1–2%) and pitch diameter approximations. For exact speeds, use timing belts or gears.