Belt Length Calculator
Calculate open-belt length from pulley diameters and centre distance in metric or imperial units.
How it works
- 1Enter the small pulley diameter, large pulley diameter, and centre distance between the two pulley shafts.
- 2Choose metric (mm) or imperial (in) — all three inputs and the result share the same unit system.
- 3The calculator applies the open-belt formula L = 2C + (π/2)(D + d) + (D − d)² / (4C) and displays the required belt length instantly.
Use cases
- Selecting the correct replacement belt for a conveyor or drive system.
- Sizing a new belt drive during machine design when centre distance is fixed.
- Verifying belt length against a manufacturer catalogue before ordering stock.
Frequently asked questions
What formula does this calculator use?
It uses the standard open-belt length formula: L = 2C + (π/2)(D + d) + (D − d)² / (4C), where D is the large pulley diameter, d is the small pulley diameter, and C is the centre distance. For example, with D = 200 mm, d = 100 mm, and C = 500 mm: L = 1000 + (π/2)(300) + (100)²/2000 ≈ 1476 mm.
Does this formula work for crossed belts?
No — the formula is for open (non-crossed) belts only. A crossed belt uses a different formula: L = 2C + (π/2)(D + d) + (D + d)²/(4C). Open belts are the standard configuration for most industrial drives.
Why does centre distance affect belt length so much?
Centre distance C appears twice in the formula — once as the two straight spans (2C) and once in the correction term for pulley size difference. Doubling C roughly doubles the belt length, while changing pulley diameters mainly affects the arc-of-contact correction.
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