Want this on your own domain?
We build this calculator as an embeddable tool — engineered, SEO- and AI-ready, delivered in 5 business days for a flat $2,800.
Get yours in 5 daysHow it's calculated
The belt wraps each pulley by a half-circle plus a correction for the size difference, joined by two straight spans equal to the center distance. The formula sums the straight lengths, the average wrap, and a small term for the diameter difference. Use consistent units throughout.
- D = 10 in, d = 4 in, C = 20 in.
- L = 40 + 21.99 + 0.45 = 62.4 in.
Reference tables
Belt length terms
| Term | Meaning |
|---|---|
| 2C | the two straight spans |
| (π/2)(D+d) | average wrap around both pulleys |
| (D−d)²/(4C) | correction for pulley size difference |
A number the tables do not give: with 10-inch and 4-inch pulleys 20 inches apart, order a belt near 62.4 inches and adjust to the nearest stock size.
See a live tool in action
The interactive version of this calculator is on our build queue. Meanwhile, these live example simulators show exactly how yours would look and behave on your own domain:
FAQ
Open or crossed belt?
This is the open (same-direction) drive. A crossed belt uses + instead of − in the size term and runs the pulleys in opposite directions.
Do I need extra for tension?
Round to the nearest standard belt and rely on the drive’s take-up (adjustable center distance or idler) for tension.
Diameter — pitch or outside?
Use the pitch diameter for V-belts and timing belts; the pitch line is where the effective length is measured.
More than two pulleys?
A serpentine or multi-pulley layout is summed span-by-span with wrap angles per pulley — this formula covers the common two-pulley case.
Build it for your catalog
Put this calculator on your domain so the engineers who run it land on your site — not a competitor's. See how it works or start the conversation.