Pulley Calculator
Calculate pulley ratios and RPM
Pulley ratio
Output RPM
Belt speed
You need to lift 500 pounds but can only pull 100 pounds. How many pulleys do you need? Your belt drive has a 4-inch motor pulley and 12-inch driven pulley—what's the speed reduction? The block and tackle shows 6 sheaves—what's the mechanical advantage?
Pulley calculations determine mechanical advantage, speed ratios, and belt lengths. Whether you're rigging lifting systems or designing belt drives, the math is essential.
What is Pulley Calculation?
Pulley calculations cover two main applications: lifting systems (mechanical advantage) and belt drives (speed/torque conversion). Both use the basic principle that pulleys trade force for distance or speed for torque.
Key formulas:
Mechanical advantage (block and tackle):
MA = Number of rope segments supporting load
Belt drive ratio:
Ratio = Driven pulley diameter / Driving pulley diameter
Output speed = Input speed / Ratio
Belt length (two pulleys):
L = 2C + π(D+d)/2 + (D-d)²/4C
where C = center distance, D = large diameter, d = small diameter
For lifting: count how many rope segments support the load. That's your mechanical advantage—ignore the rope you pull.
Why People Actually Need This Tool
A single person can lift thousands of pounds with the right pulley arrangement. Ancient civilizations built monuments with pulley systems.
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Rigging and lifting — Calculate mechanical advantage for safe lifting.
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Belt drive design — Size pulleys for speed/torque requirements.
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Theater/stage — Fly systems use counterweight pulleys.
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Garage hoists — Size systems for lifting vehicles or boats.
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Industrial conveyors — Belt length and tension calculations.
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Exercise equipment — Cable machine mechanical advantages.
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HVAC systems — Fan belt drive calculations.
How to Use the Pulley Calculator
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Select application — Lifting (MA) or belt drive (ratio).
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Enter parameters — Sheaves/pulleys for MA, or diameters for belt.
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Optional: Enter speed — For belt drive output calculation.
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View results — Mechanical advantage, ratio, or required force.
| System Type | Typical MA | Application |
|---|---|---|
| Single fixed | 1:1 | Changes direction only |
| Single moveable | 2:1 | Basic mechanical advantage |
| Gun tackle | 2:1 | Two sheaves |
| Double tackle | 4:1 | Four sheaves |
| Triple tackle | 6:1 | Six sheaves |
| Chain hoist | Variable | Ratcheting mechanism |
Theoretical MA doesn't account for friction. Real systems lose 10-15% per sheave. A 4:1 system might only give 3:1 effective.
Real-World Use Cases
1. The Engine Hoist
Context: Need to lift 600 lb engine. Can comfortably pull 100 lbs.
Problem: What pulley system needed?
Solution: MA needed = 600/100 = 6:1. Use triple block (6 rope segments).
Outcome: Engine lifted safely within strength limits.
2. The Belt Drive
Context: Motor: 1750 RPM with 3" pulley. Need 500 RPM output.
Problem: What driven pulley diameter?
Solution: Ratio = 1750/500 = 3.5. Driven pulley = 3" Ă— 3.5 = 10.5".
Outcome: Correct pulley sizes for speed reduction.
3. The Counterweight
Context: Theater fly system, 200 lb scenery piece.
Problem: Counterweight needed for 2:1 advantage?
Solution: With 2:1 system, 100 lbs counterweight balances 200 lb load.
Outcome: Operator easily controls heavy scenery.
4. The Belt Length
Context: 6" and 10" pulleys, 24" center distance.
Problem: Required belt length?
Solution: L = 2(24) + π(10+6)/2 + (10-6)²/(4×24) = 48 + 25.1 + 0.17 = 73.3".
Outcome: Correct belt ordered for application.
5. The Torque Increase
Context: Motor produces 10 Nm. Need 35 Nm at output.
Problem: Pulley ratio required?
Solution: Ratio = 35/10 = 3.5:1. Motor pulley 3", driven 10.5".
Outcome: Torque multiplied to requirements (speed reduced proportionally).
6. The Boat Lift
Context: 2000 lb boat, single person operation desired.
Problem: Pulley system for 50 lb max pull?
Solution: MA = 2000/50 = 40:1. Need complex block system or power assist.
Outcome: Understanding that manual lifting needs supplemental power.
7. The Exercise Cable
Context: Gym cable machine shows "2:1" ratio.
Problem: How much weight am I actually lifting?
Solution: 2:1 means you pull twice as far, half the weight. 100 lb stack = 50 lb effective.
Outcome: Understanding of actual resistance in cable exercises.
Common Mistakes and How to Avoid Them
A 4:1 mechanical advantage means pulling 4 feet of rope for every 1 foot of lift. Force is reduced, work is not.
Privacy and Data Handling
This Pulley Calculator operates entirely in your browser.
- No calculations are sent to any server.
- No design data is stored.
- No account required.
- Works completely offline.
Your engineering calculations stay private.
Conclusion
Pulleys are simple machines with powerful applications—from ancient monument construction to modern industrial drives. Understanding mechanical advantage and belt ratios enables effective system design.
This calculator handles both lifting systems (mechanical advantage) and belt drives (speed/torque conversion). Enter your requirements, get the specifications.
Simple machines. Complex possibilities.