Belt & Pulley Calculator
Give the two pulley diameters, the driver speed, and the centre distance to find the speed ratio, the driven pulley rpm, the belt’s linear speed, and the belt length you need.
Belt drive output
How the calculation works
The speed ratio is the driven diameter divided by the driver diameter, and the driven speed follows by scaling the driver speed with the inverse of the diameters. Belt speed comes from the driver pulley’s circumference and rotational speed, reported in both metres per second and metres per minute so it is easy to compare with a belt’s rated limit.
The belt length uses the open-belt geometry for two pulleys a fixed centre distance apart. All results are rounded for display; treat the belt length as a starting point and confirm against the range your tensioner can accommodate.
Frequently Asked Questions
How do pulley diameters set the speed?
The belt runs at the same linear speed around both pulleys, so a small pulley must spin faster than a large one to keep up. The driven pulley speed equals the driver speed multiplied by the driver diameter divided by the driven diameter. Doubling the driven pulley's diameter therefore halves its speed — the belt equivalent of a gear reduction.
What is belt speed and why does it matter?
Belt speed is how fast the belt itself travels, equal to π times the pulley diameter times its rotational speed. It matters because every belt has a maximum safe linear speed; run past it and the belt overheats, whips, or throws off a pulley. Belt speed also sets the power a given belt section can transmit, so it is a key check when sizing a drive.
How is the belt length estimated?
This tool uses the standard open-belt approximation: twice the centre distance, plus half of π times the sum of the two diameters, plus a small correction for the difference in diameters. It is accurate for typical drives where the centre distance is comfortably larger than the pulleys. When you buy a stock belt, allow some take-up adjustment on either side of this figure.
Does the speed ratio change the torque too?
Yes. Like any speed-reducing drive, a belt system that slows the output multiplies torque by the same factor, minus slip and friction losses. A belt drive is generally a little less efficient than gears because of small amounts of creep and slip, but for most workshop and light-industrial uses the ratio from the diameters is a reliable guide.
Educational tool only. Results are geometric estimates rounded for display; verify any figure that matters for design or safety against the belt and drive manufacturer’s data.