Belt Length Calculator
Use our belt length calculator to calculate belt length for a two-pulley system from large pulley diameter, small pulley diameter, and center distance. Includes the two pulley belt length formula, a worked example, and tips on how to measure a belt length.
What Is Belt Length for Pulleys?
Belt length is the total loop length of a belt that runs around pulleys. For a basic two-pulley system, belt length depends on the diameters of the pulleys and the center distance between them.
This calculator estimates belt length for an open belt drive (standard belt path) using large pulley diameter, small pulley diameter, and pulley center distance (all in inches).
If your system uses an idler or a third pulley, belt length must be calculated differently. This tool is for the two-pulley belt length formula.
Two Pulley Belt Length Formula
A common approximation for open belt length uses two straight belt runs plus pulley arc lengths, adjusted by the pulley diameter difference.
D = large pulley diameter (in), d = small pulley diameter (in), C = center distance (in).
This is a standard two-pulley open belt approximation.
How to Calculate Belt Length
- 1
Enter the diameter of the large pulley (in).
- 2
Enter the diameter of the small pulley (in).
- 3
Enter the pulley center distance (in).
- 4
The calculator applies L = 2C + (π/2)(D + d) + ((D − d)^2)/(4C) to return belt length (in).
Frequently Asked Questions
For a two-pulley open belt drive, a common approximation is L = 2C + (π/2)(D + d) + ((D − d)^2)/(4C).
Use pulley diameters and center distance to estimate belt length, then select the nearest standard belt size and confirm you have tension adjustment.
If you have the belt, you can measure the belt’s circumference with a tape measure or wrap a string around the belt, mark it, then measure the string length.
It’s used to estimate belt length in an open belt drive where a belt runs around two pulleys with a straight run between them.
No. Adding an idler or third pulley changes the belt path. This calculator is for a two-pulley system only.