Belt Length Calculator
Enter both pulley pitch diameters and the center distance to get the belt pitch length in mm and inches, plus the speed ratio.
Open calculatorLast updated: 2026-09-15
How the calculation works
- The belt wraps two straight runs plus arcs on each pulley; the classical open-belt equation collapses that geometry into one formula.
- Use pitch diameters for V-belts (roughly the groove's effective depth) and outer diameter for flat belts.
- The speed ratio comes from the diameter ratio, identical logic to gears.
Worked example
Interpreting the result
Order the nearest standard belt designation — V-belts come in fixed pitch lengths. If the belt is significantly shorter than calculated, it won't fit around both pulleys; much longer and the tensioner may run out of adjustment. The (D−d)² term shows that large diameter differences demand longer belts for the same center distance.
Assumptions
- Open (non-crossed) belt drive, both pulleys in the same plane.
- Rigid pulleys, no belt slip or creep.
Limitations
- Timing belts use pitch (tooth) length — use the manufacturer's pitch-line calculation instead.
- Jockey/idler pulleys and serpentine paths are not modeled.
- Does not check minimum wrap angle (keep ≥120° on the small pulley for V-belts).
Belt pitch length (mm) by pulley diameters and centre distance
Each cell is the calculator's result for an open two-pulley belt drive.
| Pulleys (small / large) | 300 mm centres | 400 mm centres | 600 mm centres | 800 mm centres |
|---|---|---|---|---|
| 100 / 200 mm | 1,079.6 | 1,277.5 | 1,675.4 | 2,074.4 |
| 150 / 300 mm | 1,325.6 | 1,520.9 | 1,916.2 | 2,313.9 |
| 200 / 400 mm | 1,575.8 | 1,767.5 | 2,159.1 | 2,555.0 |
Wrap angle on the small pulley (degrees)
How much of the small pulley the belt touches, for the same combinations.
| Pulleys (small / large) | 300 mm centres | 400 mm centres | 600 mm centres | 800 mm centres |
|---|---|---|---|---|
| 100 / 200 mm | 161 | 166 | 170 | 173 |
| 150 / 300 mm | 151 | 158 | 166 | 169 |
| 200 / 400 mm | 141 | 151 | 161 | 166 |
Wrap angle = 180° − 2 × asin((D − d) ÷ (2 × C)). A larger gap between pulleys increases the wrap on the small one.
Worked scenario: 100 mm and 200 mm pulleys on 400 mm centres
The calculator gives a belt pitch length of 1,277.5 mm (50.29 in) and a speed ratio of 2:1. Belts are made in standard lengths, so you pick the nearest one and adjust the centre distance with the motor mount until the belt is correctly tensioned.
Mistakes to avoid
- Mixing pitch length with outside length. Belt catalogues state one or the other, and they differ.
- Leaving no adjustment range. Allow the motor or idler to move so you can fit the belt and take up stretch.
- Ignoring wrap angle. A small wrap on the small pulley reduces the power the belt can carry; for V-belts, designers commonly try to keep it well above about 120°.
- Using pulley diameters of different groove profiles. The belt section must match the pulley.
Frequently asked questions
Should I measure the old belt instead?
A worn belt has stretched — measuring it overestimates. Use the geometry calculation, then select the closest standard belt; tensioners absorb the small difference.
What is pitch diameter?
For V-belts it's the effective diameter at the belt's neutral axis in the groove — slightly larger than the groove bottom, smaller than the outer edge. Manufacturers publish pulley pitch diameters.