Pulley Calculator

Pulley calculator finds all parameters of a belt drive system: the RPM, speed, belt length, belt tension, and torque.

Pull needed565.1403 Nideally 500 N, plus friction

Ideal mechanical advantage4×simply the number of rope sections carrying the load — count the ropes, not the pulleys

Actual mechanical advantage3.5389×88.47% overall efficiency

Efficiency lost to friction11.53%compounding 96% across 3 turns

Rope you must pull12 mto raise the load 3 m

Work done on the load6 kJ

Work you must do6.7817 kJthe difference is heat in the sheaves

With 2 rope sections1.0417 kNpulling 6 m of rope

With 6 rope sections408.811 Npulling 18 m of rope

With 8 rope sections332.6912 Npulling 24 m of rope

Where the advantage stops helpingwhen friction outgrows the gaineach extra sheave adds its own loss, so beyond a point more sections make the pull heavier, not lighter

The formula

MA = number of rope sections supporting the load

Count the ropes, not the pulleys

The mechanical advantage of a block and tackle is the number of rope sections actually supporting the load — not the number of pulleys, which is the usual mistake. A single fixed pulley has one supporting section and gives no force advantage at all; it only changes the direction you pull. Adding a movable block gives two.

As always, distance pays for force. A four-to-one tackle needs four metres of rope pulled for every metre the load rises, which is why rigging a large advantage in a confined space runs out of rope travel before it runs out of strength.

Friction is the practical limit. Each sheave loses a few percent, and those losses compound, so a ten-part tackle can lose a quarter of its theoretical advantage. Past a certain point adding sections makes the pull heavier rather than lighter — which is why real rigging rarely goes beyond about six parts without switching to a winch.