Pulley speed calculator
Enter the drive pulley diameter and speed and the driven pulley diameter. You get the driven speed, the ratio and the belt speed. Use pitch diameter where you have it; outside diameter is close enough for a rough answer.
- Driven speed
- 875 rpm
- Belt speed
- 1,833 ft/min
Ratio 2.00:1. Driven rpm equals drive diameter times drive rpm, divided by driven diameter.
Keep classical V-belts under about 6,500 ft/min and narrow-section belts under 8,000.
The pulley formulas
D1 x N1 = D2 x N2, where D is diameter and N is speed. Rearrange for whichever you need: N2 = D1 x N1 / D2, or D2 = D1 x N1 / N2.
Ratio is D2 / D1. A ratio above 1 is a speed reduction and a torque increase; below 1 is a speed increase.
Belt speed in feet per minute is pi x D x N / 12 with D in inches. It is the same on both pulleys, and it is what limits how fast a belt drive can run: classical V-belts are happiest under 6,500 fpm.
Common pulley combinations from a 1,750 rpm motor
A four-pole induction motor runs close to 1,750 rpm under load. These are the shaft speeds you get from common pulley pairs on one.
| Drive pulley | Driven 3 in | Driven 4 in | Driven 6 in | Driven 8 in | Driven 12 in |
|---|---|---|---|---|---|
| 2 in | 1,167 | 875 | 583 | 438 | 292 |
| 2.5 in | 1,458 | 1,094 | 729 | 547 | 365 |
| 3 in | 1,750 | 1,313 | 875 | 656 | 438 |
| 4 in | 2,333 | 1,750 | 1,167 | 875 | 583 |
| 5 in | 2,917 | 2,188 | 1,458 | 1,094 | 729 |
