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Pulley RPM Calculator

The driven pulley's speed, the drive ratio and the torque change between two pulleys, from the two diameters and the drive RPM.

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Two pulleys and a belt trade speed for torque. A small pulley driving a big one slows the tool down and gives it more grunt; the other way round speeds it up. This works out the driven pulley's RPM, the ratio and the torque change, so you can re-pulley a drill press, lathe or shop build to land on the speed you want.

The formula

The belt moves at one speed, so the two pulleys turn in inverse proportion to their size. The driven pulley's RPM is the drive RPM times the drive diameter, divided by the driven diameter. A 2 inch pulley on a 1750 RPM motor driving a 4 inch pulley turns the tool at 875 RPM, half speed, because the driven pulley is twice the size.

Speed and torque trade off

You never get both. Gear a tool down for more torque, to bore big holes or turn hardwood, and it runs slower. Gear it up for speed, to sand or buff, and it loses torque and can stall under load. The ratio sets both at once: double the size of the driven pulley and you halve the speed and double the torque.

Belt speed and slip

The belt itself travels at pi times the drive pulley's diameter times its RPM, which matters for how fast it wears and whether it sings. Belts also slip a little, so the real tool speed sits just under the calculated figure, and further under on a glazed or loose belt. Tension the belt firm, not tight, and the slip stays small.

Multi-step pulleys

A stepped pulley, the cone of different sizes on a drill press, is just several pulley pairs in one casting. Read the diameter of the step the belt sits on at each end and work that pair. Moving the belt to a smaller driver and bigger driven step is how you drop to a low speed for a big bit, and the reverse speeds it up for small work.

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