Pulley Speed Ratio Calculator

About this calculator

In a belt-and-pulley drive, the driver and driven pulleys are linked by the same belt speed, which means their rotational speeds are inversely proportional to their diameters — the governing relationship is N₁ × D₁ = N₂ × D₂, where N is RPM and D is diameter. A smaller driven pulley spins faster than the driver, and a larger driven pulley spins slower, exactly the trade-off mechanical designers use to step speed up or down between a motor and a driven machine.

The speed ratio is simply the driven pulley diameter divided by the driver pulley diameter (equivalently the inverse of the RPM ratio), a number machinists and mechanical engineers use directly when selecting pulley sizes for conveyors, compressors, and machine tool drives. This calculator takes both pulley diameters and the driver RPM to compute the driven pulley’s RPM and the resulting speed ratio.