Pulley Calculator
Work out a pulley system’s mechanical advantage and effort force from the number of supporting rope segments, with an optional efficiency figure for real-world friction.
Pulley result
The physics behind a pulley system
A single fixed pulley just redirects a rope, so it takes the same force to lift the load — its only benefit is letting you pull down instead of up. The real force-saving comes from a movable pulley, attached directly to the load, which is supported by two or more rope segments at once. Each segment shares an equal fraction of the load’s weight, so the tension you feel in the rope is the load divided by the number of segments.
Combine fixed and movable pulleys into a block and tackle and you can multiply the mechanical advantage further — but the rope you pull always has to travel farther than the load rises, in exact proportion to the force you save. This is the conservation of work at its cleanest: less force, more distance, same total work (plus whatever friction steals along the way).
Frequently Asked Questions
How do I count the supporting rope segments?
Count every strand of rope that physically holds up the moving pulley block (the one attached to the load) — not the total length of rope in the system. A single fixed pulley has 1 supporting segment (it only changes direction). A movable pulley pulled by one rope typically has 2 supporting segments. A block and tackle with two sheaves on the moving block commonly has 4.
Why is the effort force lower than the load weight?
Each supporting rope segment shares the load equally, so the tension in the rope you pull is the load weight divided by the number of supporting segments. With 4 supporting segments, you only need to pull with roughly a quarter of the load's weight — the ideal mechanical advantage of that system is 4.
What is the trade-off for the lower effort force?
Distance and speed. To raise the load by one metre, you must pull the rope through a distance equal to the number of supporting segments times one metre. A 4:1 pulley system needs 4 metres of rope pulled for every metre the load rises — you trade rope length and pulling speed for a lighter pull.
What does the efficiency percentage represent?
Real pulley wheels and rope have friction, so the actual effort force needed is always a bit higher than the frictionless ideal. The efficiency figure (100% = no losses) scales the ideal effort up to a more realistic estimate — a well-lubricated system with quality sheaves might run at 90–95%, while a rough rope over a plain bushing could be much lower.
Educational estimate only. Real systems vary with rope type, sheave bearings, and rigging — verify load-bearing rigging with a qualified professional.