Belt Length Calculator

Calculate belt length from pulley diameters and center distance

This belt length calculator is built for a common mechanical task: estimating the belt length for an open belt drive connecting two pulleys. The decision it supports is simple: you need a reasonable belt length estimate so you can select a standard belt size (or confirm whether an existing belt is close) before you commit to ordering parts or machining mounts.

Enter the two pulley diameters and the center distance between shafts. The calculator returns the estimated belt length for an open belt arrangement, where the belt runs without crossing between pulleys. This is the standard layout for many belt driven fans, pumps, compressors, and general machinery where rotation direction stays the same on both shafts.

In addition to the belt length, the calculator provides a couple of practical supporting numbers. It estimates the wrap angle on each pulley (how much of the pulley circumference the belt contacts). Wrap matters because low wrap on the small pulley can increase slip and reduce usable power transfer. If you add an allowance percentage, the calculator also shows an adjusted belt length to account for tensioning travel, take-up, or the fact that real belt selection often involves picking the next available standard length.

Assumptions and how to use this calculator

• This calculator assumes an open belt drive, not a crossed belt. If your belt crosses, the geometry and wrap angles differ.
• Pulley diameters should be the effective pitch diameters for the belt type, not necessarily the outside flange diameter.
• The center distance should be measured shaft-to-shaft. If you only know approximate spacing, the result is still useful as a sizing estimate.
• The allowance percentage is optional and defaults to 0%. Use it to add a small extra length for adjustment range or to target a nearby standard belt length.
• Results are geometric estimates only. Real belt selection also depends on belt type, pulley groove profile, belt stretch, and manufacturer sizing tables.

Common questions

What belt types does this work for?

The geometry is the same for many belt types (V-belts, flat belts, some timing belt layouts) as long as you are using effective pulley diameters that match the belt’s pitch line. The calculator does not choose a belt profile or tooth count. It only gives a length estimate you can use as a starting point for selecting the correct belt family and standard size.

Which pulley diameter should I measure?

Use the effective diameter where the belt actually runs. For V-belts, that is typically close to the pitch diameter rather than the outside rim. If you only have a tape measure and can measure the outside diameter, the estimate can still be close, but your final belt size should be confirmed using the pulley and belt manufacturer’s reference dimensions.

Why does the center distance validation matter?

If the pulleys are too close together relative to their sizes, the belt path becomes impractical and the simplified geometry breaks down. A very short center distance usually causes extreme wrap angles, interference, or an arrangement that cannot be assembled. If you see an error, increase the center distance or re-check pulley sizes and your measurement points.

How should I pick the allowance percentage?

If you already have a tensioning mechanism with limited travel, a small allowance like 1% to 2% can be a reasonable planning value, especially when you expect to choose the nearest standard belt length. If you are matching a replacement belt and you know the existing belt length, set allowance to 0% and compare the calculated length to what is installed. If the difference is large, re-check that you used pitch diameters.

Does wrap angle affect belt length?

Wrap angle does not change the belt length formula directly, but it affects whether the design is mechanically sensible. Low wrap on the small pulley can lead to slip under load, especially for high torque applications. If the small pulley wrap is low, consider increasing the center distance, increasing the small pulley diameter, or using an idler to increase wrap. This calculator reports wrap to flag that issue early.