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.
Last 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.
Formula
n₂ = n₁ × d₁ / d₂ v = π × d₁ × n₁ / 60 (belt speed)
| Symbol | Meaning | Unit |
|---|---|---|
n₁, n₂ | Driver / driven speed | RPM |
d₁, d₂ | Driver / driven pitch diameter | mm |
v | Belt surface speed | m/s |
Worked example
A 1750 RPM motor on an 80 mm pulley driving a 200 mm pulley: driven = 1750 × 80/200 = 700 RPM. Belt speed = π × 0.08 × 1750/60 = 7.3 m/s — comfortable. The same ratio with a 50 mm driver at 3450 RPM runs 9.0 m/s, still fine.
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.
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.