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Pulley Calculator
Enter two pulley diameters and the centre distance to get the ratio, the belt length and the driven speed.
Pulley calculator
Pulley ratio: —
Driven speed: —
Belt length: —
Speed change: —
—
How to use the Pulley Calculator
- Enter the driver and driven pulley diameters in the same unit.
- Add the centre distance between the two shafts and the driver speed.
- Read the ratio, the driven speed and the open belt length.
Features
- Pulley ratio from two diameters
- Driven speed at any driver rpm
- Open belt length from the centre distance
- Runs locally, no uploads
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Frequently Asked Questions
Common questions about the free online pulley calculator.
Divide the driven pulley diameter by the driver pulley diameter. A 250 mm driven pulley with
a 100 mm driver gives a ratio of 2.5.
Belt length is twice the centre distance, plus half the pulley circumferences, plus the
diameter difference squared over four times the centres.
Driven rpm equals driver rpm times the driver diameter divided by the driven diameter, so
1450 rpm becomes 580 rpm at a ratio of 2.5.
This calculator uses the standard open belt formula, where both pulleys turn in the same
direction and the belt does not cross over.
No. Speed depends only on the two diameters, but the centre distance is needed to work out
how long the belt has to be.
Any consistent unit works. Enter both diameters and the centre distance in the same unit and
the belt length comes back in that unit.
A small pulley has less belt contact, so it grips less. Keep the wrap angle generous and
check the belt tension regularly.
No. The pulley calculations run entirely in your browser, so the sizes and speeds you enter
stay on your device.
Yes. It is completely free, needs no sign-up, and you can size as many belt drives as you
like.
Last updated: October 10, 2026