Inputs
Results
L ≈ 2C + (π/2)(D+d) + (D−d)² / (4C). For V-belts, use pitch diameter (PD), not OD.
What this calculator actually solves
Two practical problems: (a) you've got two pulleys and a center distance, you need the belt part number; (b) you've got a belt part number and the pulleys, you need the center distance. Both are 5-line textbook formulas but most calculators only do one direction.
For V-belts, you want pitch diameter, not outside diameter. The pitch diameter is smaller than OD by the belt's depth — common 5/8" V-belts have pitch 0.4" smaller than OD.
Who this is for
Maintenance techs replacing a belt that has stretched beyond its part number and want a slightly longer belt on the next go-round.
Restoration shops quoting a belt-drive conversion on a vintage engine where the original belt is no longer made.
How to read the results
Belt length is sensitive to center distance — a 1/4" error in C translates to 1/2" in belt length, and that has to be matched to a standard part number or you end up hand-cutting links.
If the calculated belt length is between two standard sizes, go up. A slightly larger belt rides deeper in the groove and will slip more easily under load.
Limitations
This is the open-belt formula. For crossed belts, the formula is different — usually L ≈ 2C + (π/2)(D+d) + (D+d)² / (4C).
For multi-pulley serpentine belts, the calculation is much more complex; we don't cover it.
Frequently Asked Questions
What's "pitch diameter"?
For a V-belt, the pitch diameter is the diameter where the belt's neutral axis runs — it's smaller than the OD by the belt's depth. A 5/8" cross-section V-belt has a pitch diameter about 0.4" smaller than OD.
My belt stretches. Now what?
Belt stretch is normal. Most serpentine belt drives have an automatic tensioner that compensates. For fixed-center drives, an idler pulley or a slightly shorter belt is the fix.
Can I use a different belt profile?
Generally no — V-belt and toothed-belt pulleys are not interchangeable. A V-belt on a toothed pulley will slip; a toothed belt on a V-pulley will skip teeth. Match the belt to the pulley.
About this tool
What this tool is for: Belt Length Calculator is a workbench reference for working mechanics, machinists, restoration shops, and serious hobbyists. It is built because the usual online calculators skip the friction loss, the sign convention, the unit conversion, or the engineering code that actually matters on the job.
Purpose and scope
Center distance from belt part-number and pulley diameters, with matched standard lengths. Confirm against the actual belt drawn in place after installation.
How to use the body of this page
The sections above this footer — “What this calculator solves,” “Who this is for,” “How to read the results,” and “Limitations” — describe in detail what the formula does, why a tech would use it, how to interpret the number, and where the model is wrong. The FAQ block answers the three or four questions most asked about this specific tool. Together they make up the “About” content for this page; you do not need to look elsewhere.
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