Mechanical calculator
Pulley Ratio and Speed Calculator
Calculate driven speed, pulley ratio, belt travel and the theoretical change in torque.
Formula
Driven speed = driver speed × driver diameter ÷ driven diameter
A larger driven pulley slows the output and increases theoretical torque. A smaller driven pulley does the opposite.
Common mistakes
- Reversing the driver and driven pulleys.
- Using radius for one pulley and diameter for the other.
- Ignoring belt slip and efficiency losses.
- Using outside diameter when the belt system is specified by pitch diameter.
The ratio is not the whole machine
This does not determine belt type, belt tension, shaft load, pulley strength, guarding or safe operating speed.