Case 1
- Centre distance
- 300mm
- Large pulley diameter
- 120mm
- Small pulley diameter
- 80mm
915.49mm
Open with these valuesBelt Length Calculator
915.49mm
Result: 915.49 mmAn open belt is two straight runs plus the wrap around both pulleys: L = 2·C + (π / 2)·(D + d) + (D − d)² / (4·C). Keep all three measurements in one unit and the answer comes back in it. Which diameter you call the large one does not matter — the difference is squared.
Worked examples
Case 2
- Centre distance
- 200mm
- Large pulley diameter
- 100mm
- Small pulley diameter
- 60mm
653.33mm
Open with these valuesCase 3
- Centre distance
- 500mm
- Large pulley diameter
- 250mm
- Small pulley diameter
- 100mm
1,561.03mm
Open with these valuesHow it's calculated
L = 2·C + (π / 2)·(D + d) + (D − d)² / (4·C)
- StepMeasure the centre distance between the two pulley shafts.
- StepMeasure both pulley diameters in the same unit.
- ResultRead the belt length and pick the nearest standard belt at or above it.
Reference table
| Centre distance, large, small | Drive | Belt length |
|---|---|---|
| 100, 50, 50 | Equal pulleys, close together | 357.08 |
| 150, 80, 80 | Equal pulleys, one-to-one | 551.33 |
| 200, 100, 60 | Small workshop drive | 653.33 |
| 300, 120, 80 | Typical two-pulley drive | 915.49 |
| 300, 200, 100 | Two-to-one, short centres | 1079.57 |
| 500, 250, 100 | Wide reduction, long centres | 1561.03 |
Questions
How do I calculate belt length for two pulleys?
Use L = 2·C + (π / 2)·(D + d) + (D − d)² / (4·C), where C is the centre distance, D the large diameter and d the small one. Keep all three in the same unit. Pulleys 300 mm apart with 120 mm and 80 mm diameters need about 915.49 mm of belt.
What is the centre distance?
The straight-line distance between the two pulley shafts, measured centre to centre. It sets the length of the two straight runs and is the single biggest driver of total belt length.
Does this work for a crossed belt?
No. This is the open-belt formula, where both pulleys turn the same way and the belt runs straight between them. A crossed belt figure-eights to reverse the rotation, is longer, and needs the sum of the radii rather than the difference.
Does it matter which diameter is the large one?
No. The difference (D − d) is squared, so swapping the two diameters gives exactly the same belt length. Enter them in either order.
What if both pulleys are the same size?
Then the difference term vanishes and the length simplifies to L = 2·C + π·D — the two straight runs plus one full circle of wrap shared between the pulleys. Equal 50 mm pulleys 100 mm apart need 357.08 mm.
How exact is the result?
The formula is the standard approximation, not the exact geometry; the exact form replaces the squared term with an arc cosine. For the usual case, where the centre distance is larger than the big pulley, the two agree to a fraction of a millimetre.
Sources and last check
Information, not professional advice.