Pulley & Belt Length Calculator
The belt length to order for a two-pulley drive — plus the speed ratio, output RPM and belt speed that come with it. Open-belt geometry from the two diameters and the center distance.
The Belt Length You Actually Order
For a standard open-belt drive between two pulleys, the belt length follows from the two diameters and the distance between their centers:
where C is the center distance, D the large pulley diameter and d the small one. The first term is the two straight runs, the middle term wraps the belt halfway around each pulley, and the last term corrects for the slightly longer reach to the bigger pulley. The result is a pitch length — pick the nearest standard belt at or just above it.
Speed Ratio and Belt Speed
Diameters set the speed trade exactly the way gear teeth do: the ratio is D / d, and the driven pulley turns at the driver speed times the diameter ratio. Belt speed — the same all the way around — is π times the driver diameter times its RPM, and it drives how much power the belt can carry.
Mind the Wrap Angle
Grip depends on how far the belt wraps the small pulley. With unequal diameters that angle drops below 180 degrees, and once it falls under roughly 120 degrees the belt is prone to slip. Increasing the center distance or adding an idler pulley restores wrap.
Frequently Asked Questions
Is this for V-belts or flat belts?
The length geometry is the same for both. V-belts are quoted by a pitch or effective length, so match the calculated figure to the manufacturer pitch length.
What about a crossed belt?
This tool covers the common open (non-crossed) arrangement where both pulleys turn the same way. A crossed belt uses a different, slightly longer formula.
Which pulley should drive?
Set the driver to whichever pulley is on the motor. Driving the small pulley steps speed down at the large one; driving the large pulley steps it up.
Related calculators
- Gear Ratio Calculator — the same speed-torque trade with gear teeth.
- Torque Calculator — the torque a drive transmits.
- Engineering Unit Converter — length, speed and power units.
- All engineering calculators — the full library.