What is the belt length formula?

For an open belt: L = 2C + π(D+d)/2 + (D−d)²/(4C), where C is center distance, D is the large pulley diameter, and d is the small pulley diameter. For a crossed belt the (D−d) term becomes (D+d).

What units should I use?

Any consistent unit works — inches, millimeters, centimeters, etc. The result will be in the same unit you enter. Do not mix units between fields.

What is the difference between open and crossed belts?

An open belt makes both pulleys rotate in the same direction. A crossed belt makes them rotate in opposite directions and uses a slightly longer belt because the belt wraps more of each pulley.

Why does the center distance matter?

Longer center distances require proportionally longer belts. The formula also uses center distance to calculate the tangent (straight) run length and the arc of contact on each pulley.

Can I use this for V-belts?

Yes. The formula gives the pitch-line length. V-belt manufacturers specify belts by pitch length, so the result matches standard belt sizing charts directly.

Belt Length Calculator

Name: Belt Length Calculator
Availability: InStock
Author: Nham Vu

Enter two pulley diameters and the center distance to instantly calculate the required belt length.

Pulley & Belt Parameters

Belt Type

Open belt (same direction) Crossed belt (opposite direction)

Large Pulley Diameter (D)

Small Pulley Diameter (d)

Center Distance (C)

Unit

Results

Enter values and click Calculate to see results.

Formula Reference

Open Belt:

L = 2C + π(D+d)/2 + (D−d)²/(4C)

C = center distance • D = large pulley dia. • d = small pulley dia.

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Summary

Enter two pulley diameters and the center distance to instantly calculate the required belt length.

How it works

Enter the diameter of the larger (driver) pulley.
Enter the diameter of the smaller (driven) pulley.
Enter the center-to-center distance between the two pulleys.
Select whether the belt is open or crossed.
Click Calculate to see the required belt length and wrap angles.

Use cases

Sizing replacement V-belts or flat belts for machinery.
Designing conveyor and drive systems in workshops.
Engineering power transmission systems with custom pulley setups.
Checking belt specifications when modifying existing equipment.
Educational use for mechanical engineering coursework.
HVAC and agricultural equipment belt replacement.

Frequently Asked Questions

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Last updated: 2026-05-23 · Reviewed by Nham Vu