Pulley Size & RPM Calculator
Find the driven pulley speed or the pulley size needed to reach a target RPM.
Inputs
Formula
- D1 × N1 = D2 × N2
- N2 = D1 × N1 ÷ D2
- Belt speed = π × D × N ÷ 60000 (m/s)
In a belt drive the rim speed of both pulleys is the same, so diameter and speed are inversely related. A small pulley driving a big one slows the machine down and increases torque, which is exactly how most workshop machines are geared.
Step-by-step calculation
- 1
Output speed
D1 × N1 ÷ D2
576 RPM
- 2
Belt speed
π D N / 60
7.54 m/s
Worked example
- 1A 100 mm pulley on a 1,440 RPM motor driving a 250 mm pulley.
- 2N2 = 100 × 1440 ÷ 250 = 576 RPM.
- 3Belt speed = π × 0.1 × 1440 ÷ 60 = 7.54 m/s.
Tips
- Keep belt speed below 25 m/s for standard V-belts.
- A ratio above 6:1 on a single belt drive is unreliable — use two stages.
Frequently asked questions
How do I slow a machine down?
Fit a larger driven pulley or a smaller motor pulley — speed falls in proportion to the diameter ratio.
Does the belt affect the speed?
Only slightly; V-belts slip about 1–2% under load.
What about torque?
Torque changes inversely with speed — halving the speed roughly doubles the torque available.
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