Pulley & Belt Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
Enter Values
How to use this calculator
- Enter the driver and driven pulley diameters (mm).
- Optionally add the driver speed to get the driven speed, and the centre distance to estimate belt length.
- 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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Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.