Pulley Ratio Lab
Choose what you want to make, then enter only the measurements you know. The calculator can size a driver, a driven pulley, or an O-ring belt and create a printable pulley that fits your axle.
Runs in your browser with Rust and WebAssembly. Your measurements stay on your device.
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What the numbers mean
A larger driven pulley or gear lowers output speed and raises ideal output torque. A smaller driven part raises speed and lowers ideal torque. The calculator uses pitch diameter for pulleys and tooth count for gears. If you only have outside pulley diameter, treat the result as an estimate because the belt rides below the outside edge.
About the STL
The downloaded model has a raised hub, thin web, reinforced rim, and rounded groove based on the O-ring cross-section. The displayed STL bore includes your selected print clearance. Measure the printed bore before assembly because printers and materials can change hole size. The model does not add a keyway, set screw, or bearing seat.
Choosing an O-ring
Measure the round cord thickness to get the O-ring cross-section. Measure from the centre of one axle to the centre of the other for axle centre distance. The recommended O-ring result is written as inside diameter (ID) × cross-section. It includes the stretch percentage you enter; 5% is only a starting estimate, so check the O-ring supplier’s allowed stretch.
Build safely
A ratio calculator cannot certify a rotating part. Check shaft speed, belt profile, alignment, balance, print orientation, material strength, heat, load, guards, and safe failure containment. Do not use a printed prototype where failure could injure someone.