The screw shown in the Figure is actually a lever attached to a circular inclined plane. The lever part of these screws is a screwdriver.
The distance between screw threads is called pitch and has the symbol P.
L is the radius of the screw head surface. The effort is applied at the screw head surface with a turning effect using a screwdriver and as a result, the load or thread of the screw moves ahead.
The mechanical advantage of a screw is obtained by taking the ratio of the effort movement and the load movement. Hence, the mechanical advantage of a screw is the ratio of the circumference of the screw head to the distance traveled by the thread after each revolution(pitch). In other words, IMA = 2*pi*L / P, where L is the radius of the screw head surface and P is the distance between screw threads(or pitch).
Read about other types of simple machines and their mechanical advantage here: inclined plane, lever, wedge, pulley, and wheel and axle.