Pulley System Calculator
Enter pulley diameters (and optional input RPM/torque) to get the ratio, output speed, belt-driven torque and the belt surface speed check.
Last updated: 2026-09-28
How the calculation works
- Surface speed at both pulleys is equal — the ratio inverts the diameter ratio.
- Torque multiplies by the ratio less ~5% belt efficiency (slip and hysteresis).
- Belt speed checks the centrifugal limit: V-belts live below ~25–30 m/s.
Formula
ratio = D_driven / D_driver n₂ = n₁ / ratio T₂ ≈ T₁ × ratio × 0.95
| Symbol | Meaning | Unit |
|---|---|---|
D | Pulley (pitch) diameter | mm |
n | Rotational speed | RPM |
T | Torque | N·m |
Worked example
Interpreting the result
Belt drives are the quiet, cheap, forgiving transmission: they slip under overload (a feature, not a bug), absorb vibration and cost less than gears — at the price of a few percent efficiency and belt maintenance. Use pitch diameters for the math; the outer diameter lies on V-belts. Belt speed's centrifugal ceiling is why very fast drives move to synchronous belts or chains.
Assumptions
- Pitch diameters entered; ~5% belt-drive efficiency.
- No gross slip or belt creep beyond design.
Limitations
- Does not size the belt (section and length — see the belt length calculator).
- Multi-stage drives compose: multiply stage ratios.
Frequently asked questions
How do I calculate pulley speed ratio?
Driven speed = driver speed × driver diameter ÷ driven diameter. A 100 mm pulley driving a 250 mm pulley at 1750 RPM gives 700 RPM.
How fast can a V-belt run?
About 25–30 m/s of belt speed before centrifugal lift unloads the contact face. Beyond that, use synchronous belts or different drives.