🪢 Pulley System Calculator

For a block-and-tackle system of fixed and movable pulleys, enter the number of rope strands supporting the load (S) and the load weight to get the mechanical advantage (MA=S) and the actual force needed to lift it.

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Mechanical Advantage (MA)

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Force required

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What is a Pulley System's Mechanical Advantage (MA)?

The Ideal Mechanical Advantage (IMA) of a block-and-tackle pulley system equals the number of rope strands (S) actually supporting the moving load. In other words, MA = S.

A fixed pulley only changes the direction of the force — it does not reduce the force needed. Only movable pulleys reduce the required force, and the advantage grows with how many strands of a single continuous rope wrap around the movable pulley(s).

Formula for Required Force

In the ideal case (no friction), the required force is:

F = Load ÷ MA

In practice, friction between the rope and pulleys means efficiency (η) is below 100%, so:

F = Load ÷ (MA × η)

For example, lifting a 100 kgf load with S=4 (a luff tackle) at 80% efficiency requires F = 100 ÷ (4 × 0.8) = 31.25 kgf of force.

Common Pulley Configurations

These are standard textbook simple-machine examples — actual S depends on how the rope is actually routed, so counting the strands directly on your setup is the most accurate approach.

· 1 fixed pulley: S=1 (direction change only, no force gain)
· 1 movable pulley alone: S=2
· 2-pulley system (1 fixed + 1 movable): S=2
· Gun tackle (2 movable pulleys): S=3
· Luff tackle: S=4

Frequently asked questions

Is mechanical advantage (MA) always equal to the number of rope strands?

In the ideal case (ignoring friction), yes — MA = S, where S is the number of rope strands supporting the moving load (movable pulley). In practice, friction lowers the real advantage, which this calculator corrects for via the efficiency (%) value.

Does using several fixed pulleys reduce the force needed?

No. Fixed pulleys only change the direction of the force, not its magnitude (advantage). You always need at least one movable pulley to actually reduce the force.

How do I choose the efficiency (%) value?

It depends on the pulley type (bearings or not), rope material, and maintenance condition. Well-maintained ball-bearing pulleys are often estimated at 90-98%, ordinary sheave blocks around 80-90%. For exact figures, check the manufacturer's specification.