How to calculate mechanical advantage

The lever is one of the so-called "simple machines" from which many more complex machines are derived. With a lever, one can obtain a multiplication of force, but of course not a multiplication of energy. The multiplication of force can be seen to arise from the equilibrium of torques, where an input force Fe with a long lever arm Le can balance a larger resistance force Fr with a short lever arm Lr.

http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/lever.html