Zerdly

Pulley & Belt Calculator

Computes driven speed, belt length and centre distance from pulley diameters and drive speed.

100% FreeRuns in your browserIntermediate
How to use this calculator

Enter Values

Minimum 0

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the driver and driven pulley diameters (mm).
  2. Optionally add the driver speed to get the driven speed, and the centre distance to estimate belt length.
  3. Read the speed ratio, driven speed and belt length.

How it works

Belt drives behave like gears by diameter: driven speed N₂ = N₁ · D₁ / D₂, and the speed ratio is D₂ / D₁.

The open-belt length is estimated as L ≈ 2C + π(D₁ + D₂)/2 + (D₂ − D₁)² / 4C, where C is the centre distance between the two pulley shafts.

Worked example

100 mm and 300 mm pulleys, 500 mm centres. Ratio 3:1; a 1450 rpm driver turns the driven pulley at about 483 rpm; belt length ≈ 1648 mm.

Tips

  • Bigger driven pulley = slower output with more torque, same as a gear reduction.

Frequently asked questions

Does belt thickness matter?

Speeds use the pitch diameters, so use those where you can. The belt-length formula is for a standard open (non-crossed) belt; vee and timing belts are then matched to the nearest standard size.

Can I use these results for final design?

No. These calculators are for first-pass sizing, checking and learning. They assume ideal supports, static loads and elastic behaviour, and they don't apply load factors, member capacity, buckling or connection checks. Always verify against the relevant design code (e.g. AS 4100, AS 1170, Eurocode) and have a qualified engineer sign off structural work.

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Cite this tool

Zerdly. (2026). Pulley & Belt Calculator. Retrieved from https://www.zerdly.com/tools/pulley-belt-calculator/

Tip: Enter any known values to calculate the remaining results.

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