About the Pulley System Calculator
This pulley system calculator works out how hard you have to pull to lift a load with a single fixed pulley, a single movable pulley or a block and tackle with any number of supporting rope segments. Enter the mass of the load, the number of strands and an efficiency, and it returns the pull force, the ideal and actual mechanical advantage, the tension in each supporting strand and how much rope you need to pull to raise the load a given height.
It is handy for rigging, sailing, theatre fly systems, workshop hoists, rescue and tree-work haul systems, and for physics students learning about simple machines. The table compares the pull needed with different numbers of strands so you can choose a sensible setup.
Mechanical advantage equals the number of rope segments holding up the moving block — not the number of pulleys. Each sheave adds friction, so real systems need more pull than the ideal figure; typical blocks with good bearings lose a few percent per sheave, so overall efficiencies of 70–90% are common. The calculation is static (lifting at a steady speed) and ignores the weight of the rope and moving block; never exceed the rated working load of rope, blocks and anchors.
With the default inputs, the pull force needed is 288.43 N. Change any value above to recalculate instantly.
How to use the pulley system calculator
- 1Choose a single fixed pulley, a single movable pulley or a block and tackle.
- 2For a block and tackle, count the rope segments running to the moving block.
- 3Enter the load mass and how high you need to lift it.
- 4Set an efficiency (100% for an ideal answer, 80–90% for typical blocks).
- 5Read the pull force, mechanical advantage and how much rope you will haul.
Formula and method
The load’s weight is its mass times standard gravity (9.80665 m/s²). In a pulley system the moving block is held up by n rope segments that all carry the same tension, so in the ideal case each carries W ÷ n and that is also the force you pull with. The ideal mechanical advantage is therefore n — the number of supporting strands, not the number of wheels.
Friction in the sheaves means you must pull harder: the actual pull is W ÷ (n × η), where η is the overall efficiency, and the actual mechanical advantage is n × η. Energy is conserved, so to raise the load by h you must pull n × h of rope. The model ignores the weight of the rope and the moving block and assumes a steady, slow lift.
- W
- Weight of the load (N)
- n
- Rope segments supporting the moving block
- η
- Overall efficiency (fraction)
- F
- Pull force needed
- h
- Height the load rises
Worked examples
100 kg on a 4-strand block and tackle, 85% efficient
The load weighs 100 × 9.80665 = 980.7 N. Four strands share it, so the ideal pull is 245.2 N; at 85% efficiency you need 245.2 ÷ 0.85 ≈ 288.4 N (about 29 kg-force) and must pull 4 m of rope to raise it 1 m.
50 kg with a single fixed pulley
A fixed pulley has a mechanical advantage of 1, so you pull the full weight, 50 × 9.80665 ≈ 490.3 N — it only lets you pull down instead of lifting up.
300 kg on a 6-strand system, 75% efficient, lifted 2 m
The 2,942 N load spread over 6 strands is 490.3 N each. With 75% efficiency the pull rises to about 653.8 N, and lifting 2 m means hauling 6 × 2 = 12 m of rope.
Frequently asked questions
How do you calculate the mechanical advantage of a pulley system?+
Count the rope segments that support the moving block (the block attached to the load). That number is the ideal mechanical advantage. If the rope end you pull also comes off the moving block, count it too.
Does adding more pulleys always make lifting easier?+
More supporting strands reduce the pull, but each extra sheave adds friction and the rope you have to haul grows in proportion. Beyond about 4–6 strands, friction losses and rope length usually outweigh the gain for hand hauling.
Why does a single fixed pulley not reduce the force?+
A fixed pulley only redirects the rope, so the tension in the rope equals the load on both sides. It makes lifting more convenient (pulling down, using body weight) but the mechanical advantage is 1.
What is a typical pulley efficiency?+
A good ball-bearing block loses roughly 3–5% per sheave, while plain bushings or carabiners used as pulleys can lose 20–50%. Multiply the per-sheave efficiencies to estimate the overall figure.
How much rope do I need to pull?+
Rope pulled equals the lift height times the mechanical advantage. A 4:1 system lifting a load 3 m means pulling 12 m of rope, plus the fixed length needed to rig the blocks.