Belt Length Calculator
Find the belt length needed between two pulleys from their diameters and centre distance.
Inputs
Formula
- L = 2C + π/2 × (D + d) + (D − d)² ÷ (4C)
- Wrap angle = 180° ± 2 × arcsin((D − d) ÷ 2C)
An open belt runs around both pulleys plus the two straight spans. The standard formula adds twice the centre distance, half the circumference of both pulleys and a small correction for the size difference. Check the wrap angle on the small pulley too — below about 120° the belt starts slipping.
Step-by-step calculation
- 1
Straight spans
2 × C
1,200 mm
- 2
Arc allowance
π/2 × (D + d)
550 mm
- 3
Correction
(D − d)² ÷ 4C
9.4 mm
Worked example
- 1100 mm and 250 mm pulleys at 600 mm centres.
- 2L = 1200 + 1.5708 × 350 + (150)² ÷ 2400 = 1200 + 549.8 + 9.4 = 1,759 mm.
- 3The nearest standard V-belt is A69 (1,753 mm).
Tips
- Allow a 25–50 mm take-up on the centre distance so the belt can be tensioned.
- Replace matched belts as a set — a new belt beside a stretched one carries all the load.
Frequently asked questions
How do I measure an existing belt?
Read the code on its side (e.g. A42) or wrap a string around the installed belt path.
What wrap angle is safe?
Keep at least 120° on the smaller pulley; use an idler if the layout gives less.
Should I use the inside or pitch length?
This formula gives the pitch length, which is what V-belt catalogues list.
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