Ideal mechanical advantage is the force multiplication of a simple machine in the hypothetical absence of friction and deformation. The formulae for ideal mechanical advantage assume perfect component rigidity and hardness and an absence of friction.
The formulae account only for the physical dimensions of the simple machine. They define the upper performance limit that the device could achieve. The assumptions of an ideal machine having an ideal mechanical advantage go against the second law of thermodynamics, which implies that energy exchange processes must be accompanied by the loss of some energy in the form of dissipated heat. The power going into such a machine equals the power coming out of the machine, for an efficiency of 100 percent.
This constant power is equal to the product of the force applied to the machine and the velocity of the machine component movement. The ideal mechanical advantage is calculated as the ratio between the machine input force to the machine output force. This ratio is equal to the inverse of the ratio between the input velocity to the output velocity. The speed ratio of a machine can be calculated from its physical dimensions, such as the size and number of teeth of a gear.