Block & Tackle Mechanical Advantage Calculator
A block-and-tackle lets one person move a load many times their strength by reeving a rope back and forth between blocks. This calculator gives the mechanical advantage from the number of supporting rope parts, the effort you actually need (including sheave friction), and how much rope you have to haul to lift the load a given height.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Count the rope parts that support the moving block — that is n, the mechanical advantage.
- Enter the load W in kilograms.
- Optionally set an overall efficiency below 100% to allow for sheave friction, and a lift height to get the total rope you must pull.
How it works
Ideal mechanical advantage equals n, the number of parts supporting the moving block. Ideal effort is Load / n; with friction the real effort is Load / (n × efficiency). Because force is traded for distance, the rope you pull equals n metres for every 1 metre the load rises, so total rope pulled = n × lift height.
Worked example
Worked example. A 4-part tackle lifting 1000 kg has a 4:1 advantage and needs about 250 kg of effort at 100% efficiency (277.78 kg at 90%). To raise the load 1 m you pull 4 m of rope.
Common mistakes
- Counting the hauling line that leaves the fixed block as a supporting part — only parts running to the MOVING block add to the mechanical advantage.
- Assuming the ideal effort — real sheaves lose roughly 4% each, so a many-sheave purchase can need noticeably more pull than Load / n.
- Forgetting the distance penalty: high MA means hauling a lot of rope, which can run out of drum or deck space.
Frequently asked questions
How do I count the number of parts n?
Count the rope segments that pull up on the moving (travelling) block. A single fixed pulley that only changes direction gives n = 1 (no advantage); a gun tackle gives 2, a luff tackle 3, and so on.
What efficiency should I use?
Plain-bearing blocks lose more than roller/ball-bearing blocks. A rough estimate is 0.96 to the power of the number of sheaves, or use ~90% for a good, well-maintained tackle. Enter 100% only for an idealised, frictionless comparison.
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Tip: Enter any known values to calculate the remaining results.
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