Belt and pulley drives are one of the oldest and most reliable ways to transfer power, and they let you change speed simply by changing pulley sizes, which is exactly what this calculator helps you work out. Enter the diameter of the driver pulley, the speed it turns at in RPM, and the diameter of the driven pulley, and it returns the driven pulley's speed, the drive ratio between them, and the belt speed, all updating instantly as you change a value. The principle is beautifully simple: the belt moves at the same speed on both pulleys, so a small pulley driving a large one slows the output down, while a large pulley driving a small one speeds it up, always in proportion to the diameters. That makes pulleys the everyday tool for matching a motor's speed to whatever a machine actually needs, in everything from drill presses, lathes, sanders and table saws to fans, pumps, conveyors, compressors and countless pieces of farm and workshop equipment. The drive ratio the calculator gives you is the pulley version of a gear ratio, and it tells you not only how the speed changes but, roughly, how the torque changes in the opposite direction, so a 2 to 1 reduction halves the speed and roughly doubles the turning force. The belt speed is worth checking too, because belts have a maximum rated speed and conveyors are often specified by how fast the belt travels. Use it to size a replacement pulley, to set a machine to the right speed, or to design a drive from scratch. The formulas and a worked example are explained below.
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mm
mm
600 RPM
driven pulley speed
Drive ratio2.5 : 1
Belt speed471.24 m/min
How it works
Because the belt speed is the same on both pulleys, the driven speed is the driver speed times the driver diameter divided by the driven diameter. The drive ratio is the driven diameter divided by the driver diameter. The belt speed is the driver circumference, pi times its diameter, times its RPM, shown here in metres per minute.
Worked example
A 100 mm driver pulley turning at 1,500 RPM driving a 250 mm pulley gives a driven speed of 1,500 times 100 divided by 250, which is 600 RPM. The drive ratio is 250 over 100, that is 2.5 to 1. The belt speed is pi times 100 mm times 1,500, about 471 metres per minute.
