How is the ideal mechanical advantage of a wheel and axle calculated?

Mechanical Advantage is the value which tells us the factor by which the machine is able to convert the force applied to it to its output.

Ideal Mechanical Advantage of wheel and axle is calculated by 

IMA = Wheel Radius / Axle Radius

By virtue, the higher the computed IMA the lower the force needed to be exerted, this is the purpose for using machine, lower input force but larger output. (The smalled the axle radius compared to the wheel radius is the better setup.)