How do I calculate the driven pulley speed?

Use N2 = N1 × D1 / D2, where D1 and N1 are the driver pulley diameter and speed and D2 is the driven pulley diameter. Example: a 1750 RPM motor with a 100 mm pulley driving a 250 mm pulley gives N2 = 1750 × 100 / 250 = 700 RPM. The relation comes from the belt sharing the same linear speed on both pulleys, so the speeds are inversely proportional to the diameters.

Why does a small pulley driving a large pulley slow the load down?

Because the belt travels the same distance per minute on both pulleys, the larger pulley needs fewer turns to take up that belt, so it spins slower. A 100 mm driver at 1800 RPM linked to a 300 mm driven pulley gives 1800 × 100 / 300 = 600 RPM, a three-to-one reduction. Flip the pair (large driving small) and you get overdrive, where the driven shaft spins faster than the motor.

What is the belt linear speed and why does it matter?

Belt linear speed is how fast the belt itself travels, v = π × D × N, the circumference times the revolutions per minute. A 100 mm pulley at 1750 RPM moves the belt at π × 100 × 1750 ≈ 549,800 mm per minute, about 9.16 m per second. It matters because every belt has a rated maximum speed; run past it and the belt heats, stretches and throws off the pulley. Use the driver pulley for the figure since both pulleys share the same belt speed.

What is the drive ratio of a belt drive?

The drive ratio is D2 / D1, the driven diameter over the driver diameter, which equals N1 / N2. A ratio of 2 means the driven pulley is twice the driver diameter and turns at half the speed, a two-to-one reduction. A ratio of 0.5 means a small driven pulley spun up to double speed, an overdrive. The ratio tells you the speed change and, ignoring losses, the inverse torque change at the load shaft.

Can I find the pulley diameter I need for a target speed?

Yes. Pick the diameter as the unknown and enter the other three values. To turn a 1750 RPM motor with a 100 mm pulley into a 900 RPM load shaft, solve for D2 = N1 × D1 / N2 = 1750 × 100 / 900 ≈ 194 mm, so a 194 mm driven pulley hits the target. This is the everyday job when you have a fixed motor and need to pick the pulley pair that lands a fan, pump or spindle at the right speed.

Does belt slip change the result?

These formulas assume no slip, which is the right design starting point. Real V-belts slip roughly one to three percent under load, so the measured driven speed runs slightly below the calculated value. For a 700 RPM result expect maybe 685 to 695 RPM in practice. Slip rises with worn belts, low tension or overload, so if precision matters use a tachometer to confirm and adjust pulley sizing from there.

Spec a fan or blower to hit its rated RPM

An axial fan needs 850 RPM but your motor only comes in 1750 RPM. Set the driven speed as a known, pick the driven diameter as the unknown, and the tool tells you which pulley pair lands the fan on its nameplate speed. Punch in the motor pulley you have on the shelf and read back the exact driven diameter to order, instead of guessing and ending up with a fan that moves too much or too little air.

Slow a drill press or lathe spindle for a tougher material

Cutting stainless wants a slower spindle than cutting aluminium. With a fixed motor speed, step the driven pulley up in diameter and watch the spindle RPM drop in real time. Compare two or three pulley sizes side by side, copy the one that matches your feeds-and-speeds chart, and you have the belt position settled before you touch a wrench.

Check a belt is inside its rated linear speed

A belt rated for, say, 25 m per second will overheat and stretch above that. Enter your driver pulley diameter and motor RPM and read the belt linear speed straight off. If it reads past the rating, drop to a smaller driver pulley or a slower motor before the belt cooks itself, rather than discovering the problem from the smell of hot rubber.

Reverse-engineer an unlabelled drive on old machinery

You inherit a machine with two pulleys and no documentation. Measure both diameters and the motor RPM, solve for the driven speed, and you know what the output shaft actually turns at before powering it up. Share the URL with a colleague and the whole drive reopens exactly as you measured it, no re-keying of numbers.

Belt Pulley Speed Calculator: RPM, Ratio and Belt Speed

Driven speed N2 = N1 × D1 / D2, solve any unknown, plus belt speed and drive ratio, browser-only

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Solve for

Driver pulley diameter (D1)

Driver pulley speed (N1)

Driven pulley diameter (D2)

Driven pulley speed (N2)

Diameter and speed share any consistent units. Use mm for diameter to read belt speed in mm/min, inches for in/min.

Driven speed N2

700RPM

Drive ratio (D2 / D1)

2.5:1

reduction, the load shaft turns slower than the motor

Belt linear speed

549778.71 units/min

What this tool does

Free belt pulley speed calculator for sizing a V-belt or flat-belt drive between a motor and a load. A belt links a driver pulley (diameter D1 turning at speed N1) to a driven pulley (diameter D2 turning at N2). With no slip the belt moves at the same linear speed over both pulleys, so N1 × D1 = N2 × D2 and the driven speed is N2 = N1 × D1 / D2. Enter any three of the four quantities and the tool back-solves the fourth, so you can find a driven RPM, a needed motor speed, or the exact pulley diameter to hit a target speed. It also reports the drive ratio D2 / D1 (greater than one means speed reduction, less than one means overdrive) and the belt linear speed v = π × D × N, the number that decides whether a belt is running within its rated limit. Diameter and speed share any consistent units, so millimetres give belt speed in mm per minute and inches give in per minute. Everything runs in your browser with one-click copy and a shareable URL that reproduces the exact drive. 100% client-side, no upload.

Tool details

Input: Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 9 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Operations; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Belt Pulley Speed Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Spec a fan or blower to hit its rated RPM
An axial fan needs 850 RPM but your motor only comes in 1750 RPM. Set the driven speed as a known, pick the driven diameter as the unknown, and the tool tells you which pulley pair lands the fan on its nameplate speed. Punch in the motor pulley you have on the shelf and read back the exact driven diameter to order, instead of guessing and ending up with a fan that moves too much or too little air.
Slow a drill press or lathe spindle for a tougher material
Cutting stainless wants a slower spindle than cutting aluminium. With a fixed motor speed, step the driven pulley up in diameter and watch the spindle RPM drop in real time. Compare two or three pulley sizes side by side, copy the one that matches your feeds-and-speeds chart, and you have the belt position settled before you touch a wrench.
Check a belt is inside its rated linear speed
A belt rated for, say, 25 m per second will overheat and stretch above that. Enter your driver pulley diameter and motor RPM and read the belt linear speed straight off. If it reads past the rating, drop to a smaller driver pulley or a slower motor before the belt cooks itself, rather than discovering the problem from the smell of hot rubber.
Reverse-engineer an unlabelled drive on old machinery
You inherit a machine with two pulleys and no documentation. Measure both diameters and the motor RPM, solve for the driven speed, and you know what the output shaft actually turns at before powering it up. Share the URL with a colleague and the whole drive reopens exactly as you measured it, no re-keying of numbers.

Common pitfalls

Swapping driver and driven. The driver is the pulley the motor turns, the driven is on the load. Putting the motor speed on the load shaft flips the ratio and reports a reduction as an overdrive. Label the motor side as D1/N1 every time.
Mixing diameter units between the two pulleys. The formula only needs both diameters in the same unit, but feeding D1 in millimetres and D2 in inches multiplies the ratio by about 25.4 and ruins the result. Keep both pulleys in one unit, millimetres or inches, not a blend.
Reading pitch diameter versus outside diameter inconsistently. V-belt pulleys are rated on pitch diameter, slightly smaller than the rim you can measure. Using outside diameter on one pulley and pitch on the other introduces a few percent error, enough to miss a target RPM on a tight spindle job.

Privacy

Every calculation here, the N2 = N1 × D1 / D2 solve, the drive ratio and the belt linear speed, is plain JavaScript running in your browser tab. No diameter, speed or drive layout ever leaves the page, and nothing you type is logged. The one caveat: the shareable URL encodes your four values in the query string, so a share link pasted into chat records those numbers in the recipient server's access log. For a confidential machine spec, use the copy button and paste the text rather than sharing the URL.

FAQ

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Belt Pulley Speed Calculator: RPM, Ratio and Belt Speed

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Belt Pulley Speed Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Spec a fan or blower to hit its rated RPM

Slow a drill press or lathe spindle for a tougher material

Check a belt is inside its rated linear speed

Reverse-engineer an unlabelled drive on old machinery

Common pitfalls

Privacy

FAQ

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