Timing Belt Pulley Design Calculator - Daycounter
For example, if your small pulley is 6" diameter, and spins at 1000 RPM, and you need to find the second pulley size to spin it at 500 RPM, Enter Pulley1 Size 6, Pulley 1 RPM 1000, Pulley 2 RPM 500, and hit Calculate to find the second pulley diameter.