Pulley & Belt Calculator

Free online pulley and belt calculator to calculate speed ratio, RPM, belt length, and pulley sizes. Essential tool for mechanical engineering, conveyor systems, and motor drive calculations.

The Pulley & Belt Calculator helps you calculate speed ratios, RPM changes, and belt lengths for pulley systems. Enter pulley diameters and speeds to determine transmission ratios, output speeds, and required belt length for two-pulley systems.

What is a Pulley & Belt System?

A pulley and belt system is a mechanical power transmission method that transfers rotational motion and torque from one shaft to another using pulleys (wheels) and belts. The speed ratio is determined by the relative diameters of the driver (input) and driven (output) pulleys. These systems are widely used in machinery, conveyor systems, automotive engines, HVAC systems, and industrial equipment. They offer smooth, quiet operation with vibration dampening and overload protection through belt slippage.

How to Use the Pulley & Belt Calculator

Select what you want to calculate: speed ratio, belt length, or pulley size
Enter the driver pulley diameter (the pulley connected to the motor/input)
Enter the driven pulley diameter (the pulley connected to the load/output)
For speed calculations: enter the input RPM to find output RPM
For belt length: enter the center distance between pulleys
Click Calculate to see speed ratio, output speed, and belt length
Results show the relationship between pulley sizes and speeds

Pulley & Belt Formulas

1. Speed Ratio = Driver Diameter / Driven Diameter

2. Output RPM = Input RPM × (Driver Diameter / Driven Diameter)

3. Belt Length ≈ 2C + 1.57(D₁ + D₂) + (D₂ - D₁)² / (4C)

Where C = center distance, D₁ = small pulley diameter, D₂ = large pulley diameter

Understanding Speed Ratios

Speed-up (Overdrive): Driver larger than driven → Output faster than input

Speed reduction: Driver smaller than driven → Output slower than input

1:1 ratio: Equal pulley sizes → Same speed input and output

Example: 100mm driver, 200mm driven = 1:2 ratio = output half the speed

Common Belt Types

V-Belt: Trapezoidal cross-section, wedges into pulley grooves, most common type

Flat Belt: Simple design, high speed capability, requires crowned pulleys

Timing Belt (Synchronous): Toothed, no slippage, precise speed ratio

Round Belt: Small power transmission, textile machinery, low cost

Applications of Pulley Systems

Automotive: Engine accessories (alternator, water pump, AC compressor)
HVAC: Blower motors, fan drives, compressor systems
Manufacturing: Conveyor belts, production line equipment
Agriculture: Combine harvesters, threshers, irrigation pumps
Exercise equipment: Treadmills, stationary bikes, rowing machines
Industrial machinery: Lathes, milling machines, wood working equipment
Elevators: Lifting systems, counterweight mechanisms

Tips for Pulley & Belt Systems

Minimum center distance should be at least (D₁ + D₂) / 2 for proper belt wrap
Belt tension is critical - too loose causes slippage, too tight wears bearings
Check belt alignment regularly - misalignment causes premature wear
V-belts require 120° minimum wrap angle on smaller pulley for proper grip
Use timing belts when precise speed ratio is critical (no slippage allowed)
Multiple belts in parallel should be matched sets to ensure equal load sharing
Replace belts in sets - mixing old and new belts causes uneven loading

Design Considerations

When designing or selecting pulley systems, consider: (1) Required speed ratio and torque capacity, (2) Center distance constraints and belt length availability, (3) Belt type based on power transmission needs, (4) Environmental factors (temperature, moisture, chemicals), (5) Maintenance accessibility for belt tension adjustment, (6) Shaft alignment and bearing loads, (7) Safety guarding for moving parts. Proper belt tensioning typically allows 1-2 inches deflection at belt midpoint when pressed with moderate force.

Pulley & Belt Calculator

Driver Pulley (Input)

Driven Pulley (Output)

Belt Length