Belt Length Calculator

Calculate open-belt length and pulley wrap angles for a two-pulley drive.

Two-Pulley Belt Calculator

Enter all dimensions in millimeters to size an open belt drive.

Center Distance (mm)

Large Pulley Diameter (mm)

Small Pulley Diameter (mm)

About Belt Length

A belt length calculator estimates the pitch length needed to connect two pulleys in an open drive. Belt drives transfer rotation between shafts in machinery, fans, conveyors, pumps, workshop equipment, and vehicle accessories. Correct belt sizing matters because a belt that is too short may be impossible to install or may overload bearings, while a belt that is too long may slip even after the tensioner reaches its limit. This calculator uses pulley pitch diameters rather than outside diameters whenever those values are available. For an open two-pulley arrangement, the approximate length is two times the center distance plus half of pi times the sum of the pulley diameters, plus a correction based on the squared diameter difference divided by four times the center distance. The correction accounts for the unequal tangent paths created when one pulley is larger. The approximation is highly useful for routine drive selection when the center distance is reasonably large compared with the difference between pulley radii. Wrap angle describes how much of each pulley circumference contacts the belt. Unequal pulleys reduce contact on the smaller pulley and increase it on the larger pulley. The smaller pulley wrap is often the limiting value because less contact can reduce the friction available to transmit torque. Designers may increase center distance, add an idler, or select a different drive arrangement when the calculated wrap is inadequate for the belt manufacturer's rating. Measure between shaft centers and use consistent units for every dimension. Results are reported in millimeters because the inputs are millimeters, but the formula works with any single consistent length unit. Compare the calculated value with the manufacturer's available pitch-length series, then verify take-up travel, installation clearance, belt section, minimum pulley diameter, and service factor. Timing belts also require tooth-pitch and tooth-count checks, while V-belts require the correct pitch-diameter data. This result is therefore a practical starting point for selecting a catalog belt, not a replacement for the drive supplier's engineering tables.

Belt Length Examples

Typical open-drive calculations using pulley pitch diameters.

Inputs	Calculated Length	Application
C 500 mm, D 200 mm, d 100 mm	1,476.24 mm	Workshop fan drive
C 300 mm, D 120 mm, d 80 mm	915.49 mm	Compact conveyor drive
C 800 mm, D 300 mm, d 150 mm	2,313.89 mm	Pump drive layout

How to Calculate Belt Length

Measure the center-to-center distance between the two pulley shafts.
Enter the pitch diameter of the larger pulley.
Enter the pitch diameter of the smaller pulley in the same unit.
Select Calculate Belt Length and compare the result with available catalog sizes.

Belt Length FAQ

Should I use pulley outside diameter or pitch diameter?

Use pitch diameter whenever the belt or pulley manufacturer provides it. Outside diameter can produce a different result because the belt's tensile member rides below the outer edge.

Does this calculator work for crossed belts?

No, this calculation is for an open belt with both pulleys rotating in the same direction. A crossed belt follows different tangent geometry and needs a different length formula.

Why is the small pulley wrap angle important?

The smaller pulley normally has less belt contact and is more likely to slip. Its wrap angle helps determine how much torque the drive can transmit.

Can I use inches instead of millimeters?

The formula works with inches if every input is entered in inches. The displayed unit says millimeters, so convert the numerical result or use millimeter inputs for an unambiguous answer.

Will the calculated length match a catalog belt exactly?

Catalog belts are sold in discrete standard lengths, so an exact match is not guaranteed. Choose a nearby available pitch length only after confirming that the drive has enough adjustment or tensioner travel.