The formula for the mechanical advantage of a pulley is P = nW, where n is the number of ropes in the system, P is the force applied to the rope and W is the load. This applies when the force is in the same direction as the desired movement.
If the direction of force is opposite the movement, subtract 1 from the number of ropes. Fixed pulleys offer no mechanical advantage; they simply redirect applied force. Movable pulleys distribute load tension between multiple cables. For example, a 50-kg load lifted with two cables would require a 25-kg force equivalent on each end.