Pulley RPM & Belt Length Calculator
Calculate driven pulley speed (RPM), speed reduction ratio, and required belt length for a two-pulley V-belt or flat-belt drive system.
Results
What is it?
A pulley-belt drive transmits rotary motion from a motor (drive pulley) to a machine (driven pulley). The speed relationship is inversely proportional to pulley diameters � a larger driven pulley rotates more slowly. Belt length is determined by pulley diameters and centre-to-centre distance.
How to use
Enter both pulley diameters, the motor speed in RPM, and the centre distance between pulley shafts. The calculator gives the driven shaft speed, the reduction ratio (useful for torque calculations), and the exact belt length to order.
Example scenario
Drive pulley: 200mm at 1,450 RPM, driven pulley: 400mm, 800mm centre distance. Result: driven pulley runs at 725 RPM (2:1 reduction). Belt length � 2,257 mm. Order the nearest standard belt size (e.g., B88 in inch sizing).
Pro tip
For V-belts, order 3-5% shorter than calculated to allow for the groove engagement. Torque increases inversely with speed reduction � a 2:1 ratio doubles torque at the driven shaft (minus belt efficiency losses of ~3-5%). Always install belts in matched sets on multi-groove drives.