What is the formula for mechanical advantage?

Mechanical advantage is the amplification of force achieved by using a machine system, expressed as the output force divided by the input force. There is a difference, however, between the mechanical advantage a machine could give and the advantage it actually gives.

Ideal mechanical advantage is the mechanical advantage of a machine system when no energy is lost to the environment. Actual mechanical advantage differs due to friction, deflection and wear and is expressed as an efficiency variable multiplied by the ideal mechanical advantage. The efficiency of a machine system is defined as the ideal mechanical advantage divided by the actual mechanical advantage.