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Engineering Kit

Pulley and belt calculator

A free online pulley calculator and belt calculator for V-belt and flat belt drives. Size the driver and driven pulleys, set the centre distance, and the belt span redraws on the external tangents so you can see the wrap angle close up as the ratio opens.

Try the live demo
Belt drive schematic with a large driver pulley, a small driven pulley, the belt running on the external tangents and the centre distance dimension.

Try it in your browser

Drive the inputs and watch the geometry redraw in real time. The preview deliberately stops short of the numbers: belt length, speed ratio, pitch diameters, driven RPM and wrap angle all stay in the Android app.

Belt drive demo

Size the driver and driven pulleys and watch the belt span redraw.

200 mm
100 mm
450 mm

The belt follows the external tangents, so the wrap angle on the small pulley shrinks as the ratio opens up.

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What the pulley & belt calculator works out

Pulley Calculator is deliberately narrow: it solves one sizing problem completely, with the assumptions written down next to the result.

Belt length
for an open drive, ready to match against standard belt sizes.
Speed ratio
from the pitch diameters, with the driven RPM for a given motor speed.
Pitch diameter
solved the other way round when you need a target output speed.
Wrap angle
on the small pulley, which sets the power the drive can actually transmit.
Belt speed
to keep the drive inside the recommended range for the belt section.

Typical jobs

100% offline

Pulley Calculator for Android

The Android app returns the numerical belt length, the exact ratio, the driven speed, the wrap angle and the belt speed, in metric or imperial units, offline. No account, no sign-in, no data connection.

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