How it works
A block and tackle trades distance for force. Count the rope segments that actually support the moving block; that number is the ideal mechanical advantage, and you must pull that many feet of rope to lift the load one foot.
Every sheave the rope bends around loses a few percent to friction. The calculator compounds that loss over the number of sheaves to give the real pull force and real advantage.
Worked example
Lifting 1,000 lb with a 4-part tackle and four sheaves at 5% friction each:
- Ideal pull = 1,000 lb ÷ 4 parts = 250 lb
- Efficiency = (1 − 5%)^4 sheaves = 0.815
- Pull with friction = 250 ÷ 0.815 = 306.9 lb (actual advantage 3.26:1)
| Input | Value |
|---|---|
| Load weight | 1000 lbf |
| Rope parts supporting the load | 4 |
| Friction loss per sheave | 5 % |
| Sheaves the rope passes over | 4 |
| Result | Value |
|---|---|
| Pull force needed (with friction) | 306.9 lbf |
| Pull force (ideal, no friction) | 250 lbf |
| Actual mechanical advantage | 3.26 :1 |
| Rope pulled per foot of lift | 4 ft |
Assumptions and limits
- Rope parts counted are only those attached to the moving block; a lead line pulled from the fixed block does not add advantage but does add a sheave.
- Friction figures are typical, not measured. Real rope stiffness, block size, and rope diameter matter.
- Anchor loads are higher than the load weight: the rig pulls on its anchor with about load plus pull force.
Common questions
How do I count the mechanical advantage?
Count the rope parts running to and from the moving block. If the rope is pulled from the fixed block, that hauling part does not count; if it is pulled from the moving block, it does.
Why is my real pull more than load ÷ parts?
Friction in the sheaves and rope bending. It grows with every sheave the rope passes over.
What load does the anchor carry?
When you haul from the fixed block, the anchor carries about the load plus the pull force you apply. Rate the anchor and its hardware for that, with margin.
Sources
- Statics of block and tackle: ideal mechanical advantage equals the number of supporting rope parts.
- Per-sheave friction losses are typical values and vary with block type, rope, and load.
Updated 2026-09-30