Belt Length Calculator
Estimate open-belt length from two pulleys and their spacing.
Results
Calculated belt length
- Center distance
- 500 mm
- Pulley diameter difference
- 100 mm
- This is an approximate theoretical belt length. Actual belt selection should use the manufacturer's standard belt sizes.
This is an approximate theoretical belt length. Actual belt selection should use the manufacturer's standard belt sizes. Pitch length, inside length and outside length differ. Crossed belts, idlers, tensioners, belt stretch and installation allowance are not included.
Formula · How it works
Formula
L ≈ 2C + π(D + d)/2 + (D − d)²/(4C)For an open belt, L ≈ 2C + π(D+d)/2 + (D−d)²/(4C), where D and d are the larger and smaller pitch diameters and C is center distance. All dimensions are converted to meters before calculation.
Worked example
With 100 mm and 200 mm pulleys at a 500 mm center distance, L ≈ 1,000 + 150π + 5 = 1,476.24 mm. The diameter difference is 100 mm.
How it works
The formula adds the approximately straight belt spans and the portions wrapped around the pulleys. Swapping driver and driven diameters gives the same result. With equal diameters it simplifies to L = 2C + πD.
Common questions
Why does a small center distance show a warning?
The pulleys must not touch or overlap. The calculator rejects C ≤ (D+d)/2 and flags C < D+d for an additional clearance and wrap-angle check. The flag is a conservative proximity check, not a manufacturer's design limit.
Can I order a belt using only this number?
Match the result to the manufacturer's belt length convention and available standard sizes. Verify the actual center distance, adjustment range, belt section and pitch diameters before selection.
Does this work for crossed belts?
No. This formula applies to two pulleys with an open belt, without idlers or tensioners. Crossed belts have a different path and require a different formula.