Pulley Size Calculator

Disclaimer: While we strive to ensure the accuracy of our calculator tools, we cannot be held responsible for any damages or financial losses resulting from their use.

Calculating the correct pulley size is essential when designing or adjusting belt-driven systems. If you are working on machines, cars, or industrial tools, the size of the pulleys can impact how well they perform and how efficiently they operate. This tool helps you determine the appropriate driven pulley size based on your input speed and desired output speed.

Instead of manually applying formulas, you can get accurate results instantly. Just enter the known values, select your units, and let the calculator do the work.

How to Calculate Pulley Size

To calculate pulley size, you need to know the input (driver) RPM, output (driven) RPM, and the diameter of one pulley. Then, adjust the other pulley size to achieve the desired speed ratio between the two pulleys.

Basic Pulley Speed Formula

Driver RPM × Driver Diameter = Driven RPM × Driven Diameter

This formula is based on the principle that the belt speed remains constant on both pulleys.

Rearranged Formula (To find driven pulley size):

Driven Diameter = Driver Diameter × Driver RPM / Driven RPM

Example 1: Increase Output Torque (Slower Speed)

Given:

• Driver Pulley Diameter = 6 inches
• Driver RPM = 1200
• Desired Driven RPM = 600

Find: Driven Pulley Diameter

Driven Diameter = 6 × 1200 / 600 = 12 inches

A larger driven pulley slows down the RPM and increases torque.

Example 2: Increase Speed at Output

Given:

• Driver Pulley Diameter = 10 inches
• Driver RPM = 1000
• Desired Driven RPM = 2000

Find: Driven Pulley Diameter

Driven Diameter = 10 × 1000 / 2000 = 5 inches

A smaller driven pulley increases speed at the cost of torque.

Real-Life Example: Ceiling Fan Motor

Imagine a ceiling fan motor runs at 1500 RPM, but you want the fan blades to spin at 300 RPM for optimal airflow.

If the motor pulley is 2 inches in diameter, you’d need a larger fan pulley to reduce the speed.

Using the speed ratio (1500 ÷ 300 = 5), the fan pulley should be 5 times larger, or 10 inches in diameter.