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Pulley Size vs. RPM Calculator Online

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The Pulley Size vs. RPM Calculator serves as an essential tool for engineers, mechanics, and DIY enthusiasts who work with belt-driven machinery. This calculator allows users to determine the relationship between the size of the pulleys and the rotational speed of the system in revolutions per minute (RPM). It finds applications in various fields, including automotive, manufacturing, and mechanical design, facilitating precise adjustments to machinery for optimal performance. By inputting specific values, users can predict the speed changes in a system when modifying pulley sizes, ensuring efficiency and effectiveness in mechanical operations.

Formula of Pulley Size vs. RPM Calculator

The relationship between pulley size and RPM (revolutions per minute) is fundamental to understanding how changes in one can affect the other. This relationship can be encapsulated in a simple, yet powerful formula:

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RPM ratio = Diameter ratio N₁ / N₂ = D₂ / D₁


  • N₁ = RPM of the driving pulley (faster pulley)
  • N₂ = RPM of the driven pulley (slower pulley)
  • D₁ = Diameter of the driving pulley
  • D₂ = Diameter of the driven pulley

To utilize this formula effectively:

  1. Determine which pulley acts as the driver (higher RPM) and which is driven (lower RPM).
  2. Measure the diameter (D₁) of the driving pulley.
  3. To find the RPM (N₂) of the driven pulley, measure its diameter (D₂) and apply the values alongside the driving pulley’s RPM (N₁) in the formula.
  4. Alternatively, to ascertain the diameter (D₂) of the driven pulley for a desired RPM (N₂), insert the driving pulley’s diameter (D₁) and RPM (N₁) with the targeted RPM (N₂) for the driven pulley.
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  • The formula presupposes no belt slippage and that both pulleys are interconnected by the same belt.
  • Diameters (D₁ and D₂) should be in the same unit of measurement (e.g., inches, centimeters).

General Terms Table

Driving Pulley Diameter (inches)Driven Pulley Diameter (inches)Expected RPM of Driven Pulley (Given Driving Pulley RPM of 1000)

This table assumes the driving pulley is running at a constant speed of 1000 RPM and shows how the RPM of the driven pulley changes based on its diameter relative to the driving pulley’s diameter

Example of Pulley Size vs. RPM Calculator

Consider a scenario where you have a driving pulley with a diameter of 10 inches running at 1000 RPM and wish to determine the RPM of a driven pulley that is 5 inches in diameter. By applying the formula:

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N₂ = (D₂ / D₁) * N₁ = (5 / 10) * 1000 = 500 RPM

This calculation shows that the driven pulley would operate at 500 RPM, effectively halving the speed due to its smaller size.

Most Common FAQs

How do I choose the right pulley size for my machine?

Selecting the right pulley size depends on the desired output speed and the power transmission requirements of your machinery. Consider the input speed, the type of belt used, and the load on the driven pulley to make an informed choice.

Can the formula be used for any type of pulley system?

Yes, the formula is universally applicable to any belt-driven pulley system. Provided there is no slippage between the belt and the pulleys. The belt spans both pulleys uniformly.

What if there’s slippage in my pulley system?

Slippage indicates a need for tension adjustment or belt replacement. While the formula assumes no slippage for simplicity. Any occurrence of slippage requires addressing these mechanical issues to maintain system efficiency.

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