Pulley Ratio Calculator
Calculate driven pulley speed and ratio from driver and driven diameters.
How it works
- 1Enter the driver and driven pulley diameters in millimetres or inches.
- 2Enter the driver shaft speed in RPM.
- 3The calculator applies N₁D₁ = N₂D₂ to return the driven shaft speed and the pulley ratio.
Use cases
- Sizing belt drives for motors, conveyors, and fans.
- Matching spindle speed on lathes and drill presses.
- Designing multi-stage speed reduction systems in machinery.
Frequently asked questions
What formula does this calculator use?
It applies the belt-drive law N₁D₁ = N₂D₂, where N is shaft speed and D is pulley diameter. Rearranging gives driven RPM = (driver diameter × driver RPM) ÷ driven diameter. For example: 100 mm driver at 1750 rpm with a 200 mm driven pulley → 875 rpm.
Does a larger driven pulley turn faster or slower?
Slower. A driven pulley with a greater diameter than the driver reduces speed in proportion to the diameter ratio. This also increases torque by the same ratio (assuming no belt slip or loss).
Is the pulley ratio the same regardless of the unit system?
Yes. The ratio D₂ ÷ D₁ is dimensionless, so it is identical whether you measure in millimetres or inches. You can enter either unit and the ratio and RPM result will be the same.
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