Pulley RPM Calculator

About this calculator

This calculator finds a driven pulley's speed in a belt drive using N₂ = N₁ × D₁/D₂, where N₁ and D₁ are the driver pulley's RPM and diameter, and D₂ is the driven pulley's diameter. This follows from the belt moving at the same linear speed over both pulleys (v = πDN for each), so a smaller driven pulley must spin faster to match that shared belt speed, and a larger one must spin slower — the same principle behind gear ratios, just applied to a belt-and-pulley system instead of meshing teeth.

Mechanics and machinists use this to select pulley sizes for engines, air compressors, and conveyor drives when a target output speed is needed from a fixed-speed motor, and industrial maintenance techs use it to diagnose whether a swapped pulley has changed a machine's operating speed from spec.