Pulley Size & RPM Calculator
Driven pulley speed, drive ratio and belt speed from pulley diameters and motor RPM — works for sprockets and gears too.
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If you enter tooth counts, belt speed is not computed (needs diameters).
The Belt Drive Equation
Belt drives obey D₁ × N₁ = D₂ × N₂: the driver diameter times its RPM equals the driven diameter times its RPM. A small pulley driving a big one slows the shaft down (and multiplies torque); the reverse speeds it up. The tool gives the driven speed, the ratio, and the belt speed v = π · D₁ · N₁ ÷ 60,000 m/s — keep V-belts under ~30 m/s (classical) or ~40 m/s (narrow profile).
The same equation works for chain sprockets and gears — just enter tooth counts instead of diameters; the ratio logic is identical.
Frequently Asked Questions
My motor runs 1450 RPM and I need 725 — what pulleys?
You need a 2:1 reduction, so the driven pulley must be twice the driver: e.g. 100 mm on the motor and 200 mm on the shaft. Any pair with that ratio works; larger pairs grip better and wear slower.
Does belt slip change the result?
V-belts slip 1-3% under load, so real driven speed runs slightly below theoretical. Timing belts, chains and gears have none — the equation is exact for them.
Which diameter do I measure?
The pitch diameter — where the belt's neutral line rides — not the outer edge. For V-belts it sits slightly below the rim; catalogs list it per groove profile.
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Pulley Size & RPM Calculator