Pulley Speed Calculator

The driven pulley's speed is the driver speed times the diameter ratio: N2 = N1 × D1 ÷ D2. D1 and D2 are the driver and driven pulley diameters in the same units. Example: a 1,750 RPM motor with a 4″ driver and an 8″ driven pulley gives N2 = 1,750 × 4 ÷ 8 = 875 RPM. A bigger driven pulley always turns slower.

Enter any three of the four values — driver RPM (N1), driver diameter (D1), driven RPM (N2), driven diameter (D2) — and this calculator solves the fourth from N2 = N1·D1/D2. To size a pulley for a target speed instead, solve D2 = N1·D1 ÷ N_target. All four inversion directions are covered in the Speed Ratio tab.

Belt surface speed is the pulley circumference times its rotation rate: v = π × D × N ÷ 60, where D is the pulley diameter in meters and N is RPM — the result is in m/s. Example: a 4″ (0.1016 m) pulley at 1,750 RPM moves the belt at ≈ 9.31 m/s (≈ 1,833 ft/min). The Belt Speed tab also converts between m/s, ft/min, and km/h, and can invert v back to RPM.

For an open (uncrossed) belt: L = 2C + π(D1 + D2)/2 + (D2 − D1)²/(4C), where C is the center distance between shaft axes. Example: D1 = 101.6 mm (4″), D2 = 203.2 mm (8″), C = 609.6 mm (24″) gives L ≈ 1,702.2 mm (≈ 67.0″). Pick the nearest standard belt length and re-tension by adjusting C.

Multiply the per-stage ratios. Stage 1 gives N_mid = N1 × D1a/D2a and stage 2 gives N_out = N_mid × D1b/D2b, so the total ratio = (D1a/D2a) × (D1b/D2b) and N_out = N1 × total ratio. Example: 1,750 RPM through 3″→9″ then 4″→12″ gives 1,750 × (1/3) × (1/3) = 194.4 RPM. Two stages are the normal way to reach big reductions without tiny drivers.

They describe the same transmission but from different sides. Speed ratio (this tool) is kinematic: N2/N1 = D1/D2 — how RPM transforms. Mechanical advantage (our mechanical advantage calculator) is about force: a pulley system's MA is the number of rope segments supporting the load, not a ratio of diameters. The link between them is power: where speed ratio halves the RPM, ideal torque doubles (T2 = T1 × D2/D1) — real machines lose a little to belt friction, which is why torque isn't quite double.

They are inverse (ideally): T2 = T1 × D2/D1. Slowing down with a larger driven pulley multiplies torque by the same factor — e.g. the 1,750→875 RPM example doubles torque from ≈3.0 lb·ft to ≈6.0 lb·ft, minus a few percent lost to belt slip and flexing. Power is (nearly) conserved: P ≈ T1·ω1 ≈ T2·ω2. See also our gear ratio calculator for the toothed-gear analogue.

Use T = 5,252 × HP ÷ RPM for lb·ft, or the SI form T = P ÷ (2πN/60) for N·m. Example: a 1 HP motor at 1,750 RPM delivers 5,252 ÷ 1,750 ≈ 3.00 lb·ft (≈ 4.07 N·m). The Power & Torque tab converts between W, kW, and HP on both sides of P = T·ω.

Yes — the formulas here are ideal (no slip). Real V-belts slip roughly 1–5% under load, so the driven pulley runs slightly slower than N1·D1/D2. Belt dressing, proper tensioning, matched belt sets, and staying within the belt's power rating minimize slip. If your measured RPM is much lower, suspect a worn or undersized belt, not the math. For a sanity check, measure the driven shaft with a tachometer and compare.

Rearrange the ratio: D2 = N1 × D1 ÷ N_target for the driven pulley, or D1 = N_target × D2 ÷ N1 for the driver. Example: target 1,200 RPM from a 1,750 RPM motor with a 4″ driver → D2 = 1,750 × 4 ÷ 1,200 ≈ 5.83″. Practical tips: avoid drivers under ~3″ for standard V-belts (excessive flex losses), keep ratio per stage below ~3:1, and re-check belt length whenever you change a diameter.