Work out how fast a belt-driven shaft will turn from the motor speed and the two pulley diameters. The calculator also reports the speed ratio and the belt’s surface speed, and can allow for a little belt slip.
Re-powering an old drill press or lathe, slowing a grinder down for a buffing wheel, building a belt sander, or changing a band saw for metal: all of these come down to choosing two pulleys. Most single-phase shop motors run at about 1,725 or 1,750 RPM at full load (3,450–3,600 for two-pole motors), so the pulleys are what set the machine speed.
Enter the motor speed and both pulley diameters. Measure pulleys at the belt’s working diameter — for V-belts, that is roughly where the middle of the belt sits in the groove, a little smaller than the outside of the sheave.
The belt cannot move faster on one pulley than on the other, so the surface speed at both rims is the same. Surface speed is π × diameter × RPM, which means diameter and RPM trade off directly: double the driven pulley and the shaft turns at half speed.
N2 = N1 × D1 ÷ D2 × (1 − slip) · ratio = D2 ÷ D1 · belt ft/min = π × D1 × N1 ÷ 12
| Motor pulley | 2 in driven | 3 in driven | 4 in driven | 5 in driven | 6 in driven | 8 in driven | 10 in driven |
|---|---|---|---|---|---|---|---|
| 2 in | 1,725 | 1,150 | 863 | 690 | 575 | 431 | 345 |
| 2.5 in | 2,156 | 1,438 | 1,078 | 863 | 719 | 539 | 431 |
| 3 in | 2,588 | 1,725 | 1,294 | 1,035 | 863 | 647 | 518 |
| 4 in | 3,450 | 2,300 | 1,725 | 1,380 | 1,150 | 863 | 690 |
Rounded to the nearest RPM, no slip. Belt speed depends only on the motor pulley: 2 in → 903 ft/min, 2.5 in → 1,129 ft/min, 3 in → 1,355 ft/min, 4 in → 1,806 ft/min.
Default check. 1,750 RPM motor, 4 in motor pulley, 8 in driven pulley: 1,750 × 4 ÷ 8 = 875 RPM, a 2:1 reduction. The belt runs at 1832.60 ft/min.
Allowing for slip. The same drive with 2% slip entered: 1,750 × 0.5 × 0.98 = 857.5 RPM. Classical V-belts commonly show a percent or two of creep under load; timing belts show essentially none.
A drill press step. 1,725 RPM motor, 2½ in motor step, 5 in spindle step: 862.5 RPM. That is a sensible speed for a 1 in Forstner bit in hardwood.
Two stages for a big reduction. To get a metal-cutting speed of under 100 RPM at the drive wheel from a 1,725 RPM motor, one pair of pulleys is not enough. Run the calculator twice: motor 2 in to jackshaft 10 in gives 345 RPM; then enter 345 as the driver speed with a 2 in to 8 in second stage for 86.25 RPM. Total ratio 5 × 4 = 20:1.
Belts and pulleys are pinch points. Guard every belt drive before running it, unplug the machine before changing belts, and never adjust tension with the motor running. Check the driven machine’s maximum speed — grinding wheels and buffs carry a maximum RPM that must not be exceeded.
Once the pulleys are chosen you need a belt to join them. The length around two pulleys is the same geometry as a band saw blade: two straight runs plus the wrap on each pulley, with a small correction when the pulleys differ.
L ≈ 2C + π(D1 + D2) ÷ 2 + (D2 − D1)² ÷ (4C)
Example. A 4 in motor pulley and an 8 in driven pulley on 14 in centers: 28 + 18.85 + 0.29 = 47.14 in. Use the pitch (working) diameters, then match the result to the belt maker’s length chart, and mount the motor on a slotted plate or hinge so you can set the tension.
Drill presses and some lathes use a pair of opposed step pulleys so one belt fits every speed. For the same belt to work on each pair of steps, the sum of the two diameters stays roughly constant. Each pair is then just a separate run of this calculator: for example, on a 1,725 RPM motor, steps of 2 and 6 in give 575 RPM, 3 and 5 in give 1,035, 4 and 4 in give 1,725, and 5 and 3 in give 2,875.
Use the nameplate full-load speed. 1,800 is the no-slip synchronous speed of a four-pole motor at 60 Hz; under load it turns a little slower.
Yes, with tooth counts instead of diameters. The ratio works the same way, and there is no slip.
Ideally torque increases by the same ratio that speed drops, minus losses in the belt and bearings. Power stays the same at best.
Measure both pulleys and use the motor nameplate speed. A cheap optical tachometer is a good cross-check.