Pulley RPM Calculator

Multi-stage belt-drive math: output RPM, belt speed, open-belt length, and torque — with a live diagram.

Drive setup

Input RPM (motor / driver shaft)

Motor’s rated speed — e.g. 1750 or 3450 RPM.

Belt slip %

Timing belts: 0% · V-belts: ~2% · flat belts: ~4%. Use the presets below or enter a custom value.

Center distance (optional) (in)

Torque in, ft-lb (optional)

Leave center distance blank to skip belt length; leave torque blank to skip torque.

Share & saved setups

Setup name

Worked examples

Example 1 — single V-belt stage. Driver 4″, driven 8″, input 1750 RPM, 2% slip.
Stage ratio = 8 ÷ 4 = 2.00. Output = 1750 × (4 ÷ 8) × 0.98 = 857.5 RPM. Belt speed = π × 4 × 1750 ÷ 12 = 1,833 fpm (≈ 9.31 m/s). With a 24″ center distance: L = 48 + π(12)/2 + 16/96 = 67.02″. 10 ft-lb in → 19.6 ft-lb out.

Example 2 — two-stage timing-belt drive. Stage 1: 3″ → 6″; stage 2: 3″ → 9″; input 3450 RPM, 0% slip.
Stage 1 ratio 2.00 → 3450 × 3/6 = 1,725 RPM. Stage 2 ratio 3.00 → 1725 × 3/9 = 575 RPM final. Overall ratio = 2 × 3 = 6.00:1. Stage-1 belt speed = π × 3 × 3450 ÷ 12 = 2,710 fpm; stage-2 = 1,355 fpm.

FAQs

Does a bigger driven pulley increase or decrease RPM?

Decrease. Output RPM = input RPM × (driver diameter ÷ driven diameter). A driven pulley twice the driver’s size halves the speed (and roughly doubles torque, minus slip losses).

What is belt slip, and why do the presets differ?

Slip is the speed lost because the belt creeps on the pulley instead of gripping perfectly. Toothed timing belts don’t slip (0%). V-belts wedge into their groove and typically lose ~2%. Flat belts on smooth pulleys lose the most, ~4%.

How do I chain stages together?

The output shaft of stage 1 is the input shaft of stage 2. Enable stages 2 and 3 above and the calculator chains them automatically — overall ratio is the product of the stage ratios.

Which formula is used for belt length?

The standard open-belt formula: L = 2C + π(D+d)/2 + (D−d)²/(4C), where C is center distance, D the large pulley, d the small pulley. It assumes an open (uncrossed) belt — crossed belts need a different formula.

Why does torque go up when RPM goes down?

Power is conserved (minus slip/friction): power = torque × RPM. If a 2:1 stage halves the RPM, torque roughly doubles — minus the small slip loss.

Can I use mm for pulley diameters?

Yes — toggle “Diameters: mm” at the top. All math runs in inches internally and converts back, so diameters and belt length come back in your chosen unit.

How much slip should I assume for V-belts versus timing belts?

Assume 2–5% slip for V-belts — more for worn belts, heavy loads, or undertensioned drives. Timing (synchronous) belts have essentially 0% slip, so calculated RPM equals actual RPM. When in doubt, confirm with a handheld tachometer.

Can I stack more than two pulley stages?

Yes — each stage’s ratio multiplies into the total. Every stage adds a belt, bearings, and alignment work, so use only as many as you need. Recheck belt speed at each stage: an intermediate shaft can spin fast enough to exceed its belt’s rated speed even when the final output is slow.

Is there a maximum belt speed I should stay under?

Standard V-belts are usually rated to about 6500 ft/min; many manufacturers recommend staying under 5000–6000 ft/min for long belt life. Timing belts vary by profile — check the belt maker’s datasheet. If your calculated belt speed exceeds the rating, use a smaller driver pulley or a belt type rated for higher speeds.

Why does my calculated RPM differ from what I measure?

In order of likelihood: belt slip, input RPM differing from the nameplate (motors run slower under load), measuring outside instead of pitch diameter, and tachometer error. Check them in that order before doubting the math.

How do I choose pulley sizes I can actually buy?

Pulleys come in standard diameter and bore steps, so your exact math size may not exist. Pick the nearest stock sizes, recalculate the resulting RPM, and confirm it’s within tolerance. Between two close options, prefer the larger pair — larger pulleys bend the belt less sharply, extending belt life and reducing slip.

Limitations

Slip presets are rules of thumb, not measurements — worn, wet, or undertensioned belts slip more. The belt-length formula is for open (uncrossed) belts only and ignores belt thickness. Torque math assumes ideal power transfer minus slip; it ignores bearing friction and belt hysteresis. Always confirm critical speeds and tensions against the belt manufacturer’s tables.

A pulley RPM calculator tells you how fast the driven shaft of a belt drive will spin. Enter the driver pulley’s RPM and both pulley diameters, and it returns the driven RPM, the drive ratio, and the belt speed — a quick, reliable way to size pulleys for a motor, drill press, lathe, fan, or any belt-driven machine.

How to use the pulley RPM calculator

Enter the driver RPM. This is the speed of the motor or input shaft. Use the motor’s rated speed (e.g., 1750 or 3450 RPM) or a measured value.
Enter the driver pulley diameter. For V-belts, use the pitch diameter, not the outside diameter. Pitch diameter is measured where the belt rides in the groove.
Enter the driven pulley diameter. This is the pulley on the machine shaft you are driving.
Read the driven RPM. The calculator divides the driver diameter by the driven diameter and multiplies by the driver RPM.
Add slip (V-belts only). Real V-belts slip about 2–5%, so reduce the result by your slip percentage to get the shaft speed you’ll actually see. (Timing belts don’t slip; skip this step.)
Multi-stage drives: if the driven pulley of stage one shares a shaft with the driver pulley of stage two, feed the stage-one result in as the stage-two input RPM. The calculator compounds the ratios automatically.

Pulley RPM formulas with worked examples

Driven RPM

Driven RPM = driver RPM × driver diameter ÷ driven diameter

Example: a 1750 RPM motor drives a 4-inch pulley connected by belt to an 8-inch pulley.
1750 × 4 ÷ 8 = 7,000 ÷ 8 = 875 RPM on the driven shaft.

That’s a 2:1 reduction — half the speed, roughly twice the torque (minus belt losses).

Belt speed

Belt speed (ft/min) = π × pulley diameter (in) × RPM ÷ 12

Example: the 4-inch driver pulley at 1750 RPM.
3.1416 × 4 × 1750 ÷ 12 = 21,991.1 ÷ 12 = 1832.6 ft/min.

Standard V-belts are typically rated to about 6500 ft/min; exceeding it causes heat, rapid wear, and possible belt failure. Check belt speed whenever you change pulleys.

Two-stage compound example

Stage one: 1750 RPM motor → 4-inch driver to 8-inch driven = 875 RPM.
Stage two: a 4-inch pulley on that same shaft drives a 10-inch final pulley: 875 × 4 ÷ 10 = 350 RPM.
Overall: 0.5 × 0.4 = 0.2 → a 5:1 reduction from 1750 RPM.

Goal	Driver RPM	Driver → Driven	Result
Slow a 3450 RPM motor	3450	3 in → 9 in	1150 RPM (3:1 reduction)
Speed up a fan	1750	8 in → 4 in	3500 RPM (1:2 increase)
Gentle reduction	1750	6 in → 8 in	1312.5 RPM (1.33:1 reduction)

Common mistakes

Measuring the outside diameter of a V-belt pulley. Use the pitch diameter — where the belt rides in the groove, roughly 0.1–0.25 in smaller than the outside diameter. Using the outside diameter makes your calculated RPM slightly high.
Ignoring belt slip. V-belts slip 2–5% under load. If you size pulleys for exactly 900 RPM and the belt slips 3%, the shaft really turns about 873 RPM. Build the slip into the calculation.
Mixing up driver and driven. A bigger driver than driven speeds things up; a bigger driven than driver slows things down. Swapping them gives you the inverse of the correct answer.
Forgetting that multi-stage ratios multiply. Two 2:1 reductions in series give a 4:1 reduction, not 2:1. Work through each stage in order instead of averaging the pulleys.
Using the motor’s nameplate RPM as gospel. A motor labeled 1750 RPM often runs a few dozen RPM slower under full load. If precision matters, measure the actual input speed with a tachometer.