Science & Engineering
Block and tackle calculator.
Mechanical advantage of a pulley system equals the number of rope segments supporting the load. Get the pulling force, the rope you must haul per metre lifted, and the effect of friction. Runs in your browser.
Pulley system
Result
Fill the form and press Calculate.
Worked example
A 4-part tackle lifts an 800 N load (about 82 kgf). Ideal mechanical advantage = 4, so you pull 200 N and haul 4 m of rope for every metre the load rises. If the sheaves are 85% efficient overall, the pull rises to 800 ÷ (4 × 0.85) = 235.3 N and the working mechanical advantage drops to 3.40 : 1.
How it works
In a block and tackle the same rope runs round several sheaves, so the load hangs from several parts of it at once. The mechanical advantage of an ideal system is the number of rope parts supporting the moving block. The pulling force is the load divided by that number, and, because work in equals work out, the hauling end moves that many times as far as the load: to raise a load at 1 m/s a 4-part system needs the rope pulled at 4 m/s.
Friction breaks the ideal. Dividing by an overall efficiency η gives the effort F = W ÷ (n × η). Efficiency is 1 for an ideal frictionless system; adding more sheaves eventually stops helping because each adds friction. This page asks for the efficiency rather than assuming a value per sheave, since it varies with the pulleys and rope.
Not for safety-critical design
Planning and learning aid only. It is not for structural, lifting, pressure-system or other safety-critical design, and it is not professional engineering advice. Have a qualified engineer check anything that carries people or could fail dangerously.
Sources and basis: Wikipedia: Block and tackle; Wikipedia: Mechanical advantage. Checked 2026-10-16. Planning aid only; see the note above.
FAQ
What is the mechanical advantage of a block and tackle?
For an ideal system it equals the number of rope parts supporting the moving block. Four supporting parts give a mechanical advantage of 4.
How do I count the supporting rope segments?
Count the rope parts that run to the moving block that holds the load. A rope that only changes direction over a fixed pulley, such as the one you pull, does not add to the count.
How much rope do I have to pull?
The rope you pull equals the supporting segments times the height the load rises. A 4-part tackle needs 4 m of rope hauled to lift the load 1 m.
How does friction change the answer?
Friction in the sheaves means more pull is needed: effort = load ÷ (segments × efficiency). Enter an overall efficiency below 100% to see its effect.
Can I use this to select rope or pulleys?
No. It gives the ideal pull only. Rope, pulleys, anchors and hooks all have rated working loads that must be checked separately, with a safety margin.