This pulley RPM calculator finds the speed of a driven pulley from the motor speed and both pulley diameters, then adds the speed ratio, belt length and belt speed. It suits anyone setting up a drill press, lathe, fan, go-kart or shop machine who needs to know what speed a pulley swap will really deliver.
Center distance and target speed are optional; leave them blank if you only need the driven RPM. Assumes no belt slip and an open (uncrossed) belt. Belt length uses the standard approximation; for V-belts, measure on the pitch diameter and pick the nearest stock length.
How the Pulley RPM Calculator Works
A belt moves at one surface speed, so the rim of each pulley travels the same distance every minute. A small pulley has to spin faster to keep up with a large one. That means speed and diameter trade off exactly: double the driven pulley diameter and the driven shaft turns at half the speed. The pulley RPM calculator uses that rule for speed, adds the textbook open-belt approximation for belt length, and works out how fast the belt itself is moving, which matters for belt ratings and safety.
Formula: Driven RPM = Driver RPM × Driver diameter ÷ Driven diameter. Speed ratio = Driven diameter ÷ Driver diameter. Belt length ≈ 2 × C + π × (D1 + D2) ÷ 2 + (D2 − D1)^2 ÷ (4 × C). Belt speed = π × D1 × Driver RPM, shown in ft/min (inches ÷ 12) or m/s (mm ÷ 60,000). Driven size for a target = Driver diameter × Driver RPM ÷ Target RPM.
- Units — inches or millimetres for the diameters and center distance. Switching converts the values you typed.
- Driver pulley diameter — the pulley on the motor shaft (D1).
- Driver speed — motor RPM from its nameplate, such as 1,725 or 3,450.
- Driven pulley diameter — the pulley on the machine shaft (D2).
- Center distance (optional) — shaft center to shaft center (C), used only for belt length.
- Target driven speed — optional RPM you want at the machine; the tool returns the driven pulley size that delivers it.
Example Scenarios
| Driver Ø / RPM | Driven Ø | Center | Driven RPM | Ratio | Belt length | Belt speed | Size for target |
|---|---|---|---|---|---|---|---|
| 3 in / 1,750 | 6 in | 18 in | 875 | 2.00:1 | 50.26 in | 1,374 ft/min | 5.25 in (1,000 RPM) |
| 2.5 in / 1,725 | 8 in | 24 in | 539.1 | 3.20:1 | 64.81 in | 1,129 ft/min | 7.19 in (600 RPM) |
| 4 in / 3,450 | 2 in | 12 in | 6,900 | 0.50:1 | 33.51 in | 3,613 ft/min | 2.76 in (5,000 RPM) |
| 5 in / 1,140 | 10 in | 20 in | 570 | 2.00:1 | 63.87 in | 1,492 ft/min | 9.50 in (600 RPM) |
| 100 mm / 1,450 | 250 mm | 500 mm | 580 | 2.50:1 | 1,561 mm | 7.59 m/s | 207 mm (700 RPM) |
Pulley Size Calculator Tips for Shop Machines
The pulley ratio formula is exact for a belt that does not slip, but real V-belts creep by a percent or two under load. For most shop jobs that is close enough. The bigger errors usually come from measuring the wrong diameter.
- Measure the pitch diameter for V-belt pulleys, where the middle of the belt rides, not the outside rim. On a typical small pulley the outside diameter can be noticeably larger, which skews the result.
- Use nameplate speed for a motor pulley speed calculation. A “1,725 RPM” induction motor already includes slip; a synchronous speed of 1,800 is never reached under load.
- Very small driver pulleys shorten belt life, because the belt bends sharply around them. Check the belt maker’s minimum diameter.
- Belt length from any belt length calculator is a starting point. Choose the nearest stock size and leave adjustment in the motor mount for tensioning.
- Keep an eye on belt speed. Belt makers publish maximum speeds for each belt type, and fast belts need guards.
- Check the driven machine’s maximum safe speed before fitting a smaller driven pulley, especially on grinders, saws and fans.
For multi-step drives, run the pulley RPM calculator once per stage and feed each driven speed into the next stage as the driver speed. For belt types, slip and power transmission basics, see Wikipedia’s article on mechanical belts.
Pulley RPM Calculator FAQ
How do I calculate pulley RPM?
Multiply the driver speed by the driver pulley diameter, then divide by the driven pulley diameter. A 3-inch pulley on a 1,750 RPM motor driving a 6-inch pulley gives 875 RPM. Both diameters must use the same unit, and the result assumes the belt does not slip on either pulley.What size pulley do I need for a certain RPM?
Divide the driver diameter times the motor RPM by the speed you want. To get 1,000 RPM from a 3-inch pulley on a 1,750 RPM motor, you need 3 × 1,750 ÷ 1,000 = 5.25 inches. Enter that target in the optional field and the tool shows the size for you.Does the pulley RPM calculator work in inches and millimetres?
Yes. Use the inch or mm toggle. Speed ratios do not depend on units, so driven RPM is the same either way, while belt length and belt speed switch between inches and feet per minute, or millimetres and metres per second. Switching converts the diameters you already typed.Is there a free pulley RPM calculator app?
No download is needed. This page runs in any phone browser, so you can bookmark it and use it at the bench like an app. It works offline once loaded, stores nothing, and gives the same result as the paper formula, with belt length and belt speed added.How accurate is the belt length result?
The formula is the standard approximation for an open belt on two pulleys and is usually within a small fraction of an inch for normal drives. Differences come from belt thickness and measuring outside rather than pitch diameter. Use it to pick the nearest stock belt, then set final tension with the motor mount.What happens if I swap the two pulleys?
The speed ratio flips. A 3-inch driver and 6-inch driven halve the speed, while a 6-inch driver and 3-inch driven double it. Belt length stays the same, but belt speed changes because it depends on the driver pulley. Always check the machine can safely run at the new speed.Related Calculators
Pulley speeds are ratios, so the ratio calculator for simplifying speed ratios helps when you compare drive options. The circumference calculator for pulley rims shows how far the belt travels per turn, and the speed calculator for distance and time converts belt speed into other units after you use the pulley RPM calculator.