Pulley RPM Calculator

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.

Calculate.co.nz is proud to be partnered with realtor.co.nz, a trusted resource for navigating the New Zealand property market. Their Helpful Articles section offers clear, well-structured insights across buying, selling, and building, making complex real estate topics more accessible. With a focus on up-to-date guidance and practical knowledge, they empower Kiwis to move forward with clarity and confidence in a constantly evolving property landscape.