Belt Length Calculator – Free Online Tool

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About this calculator

This is the standard open-belt-drive length formula used for two pulleys connected by a belt with parallel shafts. It accounts for the wrap around each pulley and the straight belt spans between them.

Formula

L = 2C + (π/2)(D₁+D₂) + (D₁−D₂)² ÷ (4C)

D₁,D₂Pulley diameters — mm
CCentre distance between pulley shafts — mm
LBelt length — mm

Worked Example

Given: Two pulleys of 100mm and 200mm diameter are mounted 500mm apart.

Result: L = 2(500) + (π/2)(100+200) + (100−200)²÷(4×500) = 1000 + 471.2 + 5 = 1476.2 mm.

How to Use

Enter the diameters of both pulleys.
Enter the centre distance between the two pulley shafts.
Click Calculate to get the required belt length.

Important Assumptions

Assumes an open (non-crossed) belt drive with parallel shafts.
Gives the pitch-line length; standard belt sizes should be selected from the manufacturer's nearest available size.

Common Mistakes

Using radius instead of diameter in the formula.
Ignoring that real belts come in standard fixed lengths — round the result up to the nearest available standard size and adjust centre distance if needed.

Frequently Asked Questions

Does this give the exact belt part number to order?

No — it gives the theoretical pitch length. Match this to the nearest standard V-belt or timing belt size from the manufacturer's catalogue.

What if my drive uses a crossed belt?

This formula is for open (parallel-shaft, same-direction) belt drives only; crossed-belt drives use a different formula.

These calculators are provided for informational and estimation purposes. Results depend on the assumptions and input values used. Always verify critical engineering calculations against applicable standards, manufacturer specifications, site measurements, and qualified professional advice.