UtilibyV-Belt Length Calculator
Calculate V-belt length from pulley diameters and center distance. Free, fast and no sign-up.
How to use this calculator
- Choose Metric (mm) or Imperial (in) units.
- Enter the pitch diameter of the small pulley and the large pulley.
- Enter the center distance between the two pulley axes.
- Optionally enter the driver RPM to see the driven RPM.
- Read the belt length, wrap angle and speed ratio below.
The formula explained
For an open belt drive, the belt length is:
L = 2C + 1.57(D + d) + (D − d)² / (4C)
- L — belt length (pitch length)
- C — center distance between pulley axes
- D — large pulley pitch diameter
- d — small pulley pitch diameter
- 1.57 ≈ π/2
The formula has three parts: the two straight belt runs (2C), the arc length around both pulleys (1.57 × (D + d)), and a correction for the size difference ((D − d)² / 4C).
Wrap angle
The wrap angle is how much of the small pulley's circumference the belt contacts. It should be at least 120° for the belt to grip without slipping. Below that, increase center distance or reduce the pulley size difference.
Pitch diameter vs outer diameter
For V-belts, always use pitch diameter — the circle where the belt sits inside the pulley groove. Using outer diameter gives a belt that is too long.
Worked example
Metric: d = 100 mm, D = 250 mm, C = 500 mm.
L = 2(500) + 1.57(250 + 100) + (250 − 100)² / (4 × 500)
L = 1000 + 549.5 + 11.25 = 1560.75 mm
With 3% stretch allowance: ~1607 mm.
Frequently asked questions
What is the formula for V-belt length?
L = 2C + 1.57(D + d) + (D − d)² / (4C), where C is center distance and D, d are pulley pitch diameters.
What is a good wrap angle?
At least 120° on the small pulley. Below that, the belt may slip under load.
Should I use pitch or outer diameter?
Always use pitch diameter for V-belts. Outer diameter produces an incorrect belt length.
How much should I add for stretch?
Add 2–5% for belt stretch and tensioning adjustment.
Is anything I enter stored?
No. All calculations run in your browser.