Pulley Speed Calculator – RPM & Belt Speed
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Pulley Speed Calculator

Calculate driven pulley RPM, driver RPM, belt speed, or pulley diameter using pulley dimensions and rotational speed with automatic unit conversion.

Calculate Pulley Speed

Pulley Speed Formula N₁D₁ = N₂D₂. Belt speed: v = πDN / 60.

Speed Result

Driven Pulley Speed
0 RPM
Formula Used N₂ = N₁ × D₁ / D₂
Output Speed Behavior Waiting for calculation
Driver RPM 0 RPM
Driven RPM 0 RPM
Speed Ratio 0 : 1
Belt Speed 0 m/s
Driver Diameter 0 mm
Driven Diameter 0 mm
Driver Speed Belt Motion Driven Speed
The calculations assume an ideal no-slip belt drive. Actual output speed may differ because of belt slip, pulley wear, stretch, and operating load.

What Is a Pulley Speed Calculator?

A Pulley Speed Calculator determines rotational speed in a belt-driven pulley system. It uses the relationship between driver pulley diameter, driven pulley diameter, and shaft RPM.

When pulley diameters are different, the rotational speed changes. A larger driven pulley generally reduces RPM, while a smaller driven pulley generally increases RPM.

This calculator can find driven RPM, driver RPM, theoretical belt speed, or the pulley diameter required to achieve a target shaft speed.

Pulley Speed Calculator Formulas

Pulley Speed Relationship N₁D₁ = N₂D₂

Describes the ideal speed relationship between two belt-connected pulleys.

Driven Pulley Speed N₂ = N₁D₁ / D₂

Calculates output pulley RPM from input speed and pulley diameters.

Driver Pulley Speed N₁ = N₂D₂ / D₁

Calculates the input RPM required for a desired driven pulley speed.

Belt Speed v = πDN / 60

Calculates the theoretical linear speed of the belt.

Driven Diameter D₂ = N₁D₁ / N₂

Determines driven pulley diameter for a target RPM.

Driver Diameter D₁ = N₂D₂ / N₁

Determines driver pulley diameter from known output speed.

How Pulley Size Affects Speed

Pulley speed depends inversely on pulley diameter. If the driven pulley becomes larger while the driver stays the same, the output shaft turns more slowly.

N₂ / N₁ = D₁ / D₂

Larger Driven Pulley

A larger driven pulley produces a speed reduction. For example, if the driven pulley is twice the diameter of the driver, its theoretical RPM is half the driver RPM.

Smaller Driven Pulley

A smaller driven pulley produces a speed increase. If the driven pulley is half the driver diameter, its theoretical RPM is approximately twice the driver RPM.

How to Calculate Pulley Belt Speed

Belt speed is the linear speed of the belt around the pulley. When diameter is measured in meters and speed is measured in RPM, the belt speed equation is:

Belt Speed = π × Diameter × RPM / 60

For an ideal belt system with no slip, belt speed is approximately the same at both the driver and driven pulley.

Pulley Speed Formula Symbols

N₁ — Driver RPM

Rotational speed of the driving or input pulley.

N₂ — Driven RPM

Rotational speed of the driven or output pulley.

D₁ — Driver Diameter

Effective diameter of the driver pulley.

D₂ — Driven Diameter

Effective diameter of the driven pulley.

v — Belt Speed

Linear velocity of the moving belt.

π — Pi

Mathematical constant used to calculate pulley circumference.

How to Use the Pulley Speed Calculator

1

Select the Required Calculation

Choose driven RPM, driver RPM, belt speed, driven diameter, or driver diameter.

2

Enter RPM

Enter the known driver or driven shaft rotational speed.

3

Enter Pulley Diameter

Enter the known pulley diameters using millimeters, centimeters, inches, meters, or feet.

4

Select an Output Unit

For belt-speed and diameter calculations, select your preferred result measurement unit.

5

Calculate Pulley Speed

Click the calculate button to see the main answer, speed ratio, pulley sizes, and belt speed.

Pulley Speed Calculation Example

Suppose a motor drives a 4-inch pulley at 1800 RPM. The driven pulley has a diameter of 8 inches.

N₂ = N₁ × D₁ / D₂ N₂ = 1800 × 4 / 8 N₂ = 900 RPM

The driven pulley therefore rotates at approximately 900 RPM, assuming there is no belt slip.

Belt Speed Example

A 4-inch pulley has a diameter of approximately 0.1016 meters. At 1800 RPM:

v = π × 0.1016 × 1800 / 60 v ≈ 9.58 m/s

Pulley Speed Calculator Applications

Fans & Blowers

Estimate fan or blower shaft speed from motor RPM and pulley sizes.

Industrial Machinery

Calculate shaft RPM for belt-driven production equipment.

Mechanical Drives

Compare input and output speed in pulley transmission systems.

Workshop Equipment

Estimate speed changes in common belt-driven machines.

Engineering

Solve pulley speed, diameter, and belt velocity problems.

Frequently Asked Questions

For an ideal two-pulley belt drive, N₁D₁ = N₂D₂, where N represents RPM and D represents pulley diameter.

Driven pulley RPM is calculated as driver RPM multiplied by driver diameter and divided by driven diameter.

For the same driver conditions, a larger driven pulley reduces output RPM while a smaller driven pulley increases output RPM.

Belt speed is calculated as pi multiplied by pulley diameter in meters multiplied by RPM and divided by 60.

No. Pulley speed usually refers to rotational speed in RPM, while belt speed is the linear velocity of the moving belt.

Yes. Belt slip can make the actual driven pulley speed differ from the theoretical value predicted by pulley diameter ratio.