⚙️ Pulley Calculator
Calculate Pulley Speed and Ratio
How to Use This Calculator
Enter Driver Pulley Diameter
Input the diameter of the driver (power source) pulley in inches.
Enter Driven Pulley Diameter
Enter the diameter of the driven (output) pulley in inches.
Enter Driver RPM
Input the RPM of the driver pulley (motor speed, typically 1750 or 3450 RPM for AC motors).
Calculate
Click calculate to get the speed ratio and driven pulley RPM.
Formula
Speed Ratio = Driver Diameter / Driven Diameter
Driven RPM = (Driver RPM × Driver Diameter) / Driven Diameter
Where:
- Driver Diameter = Diameter of input pulley (inches)
- Driven Diameter = Diameter of output pulley (inches)
- Driver RPM = Speed of driver pulley (RPM)
Example Calculation
If you have:
- Driver diameter: 4 inches
- Driven diameter: 8 inches
- Driver RPM: 1800
Calculation:
Speed Ratio = 4 / 8 = 0.5:1 (2:1 reduction)
Driven RPM = (1800 × 4) / 8 = 7200 / 8 = 900 RPM
About Pulley Calculator
The Pulley Calculator is a practical tool for engineers, mechanics, and DIY enthusiasts working with belt-driven systems. Pulleys are used to transfer power and change speed between rotating shafts. This calculator helps determine the speed ratio between pulleys and calculate the output speed of a driven pulley based on input speed and pulley diameters.
When to Use This Calculator
- Machine Design: Calculate speed ratios for belt-driven machinery
- Motor Selection: Determine required motor speed for desired output speed
- Pulley Sizing: Select appropriate pulley sizes to achieve target speed ratios
- System Troubleshooting: Verify pulley ratios in existing equipment
- DIY Projects: Design pulley systems for workshops and custom machinery
Why Use Our Calculator?
- ✅ Instant Results: Get speed ratio and RPM calculations immediately
- ✅ Easy to Use: Simple interface requiring only pulley diameters and input RPM
- ✅ Accurate: Uses standard mechanical engineering formulas
- ✅ 100% Free: No registration or payment required
- ✅ Educational: Includes detailed formula explanations and examples
- ✅ Practical: Essential for belt-driven system design
Common Applications
AC Motors: Calculate output speeds when connecting motors (typically 1750 or 3450 RPM) to equipment requiring different speeds.
Machinery: Design pulley systems for lathes, drill presses, bandsaws, and other workshop equipment.
Automotive: Calculate ratios for serpentine belt systems, supercharger drives, and accessory pulleys.
Industrial Equipment: Design belt drive systems for conveyors, pumps, compressors, and manufacturing equipment.
Tips for Accurate Results
- Measure pulley diameters accurately - use the outer diameter for V-belt pulleys
- Larger driver pulley = faster driven speed (speed increase)
- Larger driven pulley = slower driven speed (speed reduction)
- For multiple pulley systems, calculate step-by-step or multiply ratios
- Consider belt slip (typically 2-5%) in real-world applications - actual speed may be slightly less
Frequently Asked Questions
What is a speed ratio?
Speed ratio is the relationship between driver and driven pulley speeds. A 2:1 ratio means the driven pulley turns at half the speed of the driver. A 0.5:1 ratio means the driven turns at twice the speed.
How do I increase or decrease speed?
To increase driven speed: make the driver pulley larger or the driven pulley smaller. To decrease speed: make the driver pulley smaller or the driven pulley larger.
Does belt type affect the calculation?
The diameter-based calculation works for all belt types (V-belt, flat belt, timing belt). However, use the effective pitch diameter for timing belts rather than outer diameter.
What about belt slip?
Belt slip typically ranges from 2-5% depending on belt tension, condition, and load. For precise applications, account for slip by reducing calculated speed by 2-5%.
Can I use this for multiple pulleys?
For multiple pulley systems, calculate each stage separately or multiply the speed ratios together. For example, two 2:1 reductions in series give a total 4:1 reduction.