A machine screw, or machine bolt, fits into a tapped, or threaded hole. It is often fastened with a nut as well. Machine screws are variable in size and form. Differences include material, length, breadth, head style, head size, thread size and slot type.
The fastener length is measured differently depending on the type of screw head. The angle on the head used to measure the length is the angle that is flush with the surface. From that edge, the length is measured to the point of the screw parallel to the shaft.
The fastener diameter has two dimensions. Thread diameter is expressed along a range from #0 to #14. #0 is a .060-inch edge-to-edge thread diameter. From zero, there are 14 increments of .013-inch each. The frequency of the twist -- or number of threads per shaft-inch -- completes the description of the thread size. For example, a 4-40 screw is a #4 thread diameter with a twist of 40 turns per shaft-inch.
Ascertain the material, sizes, length and head style to determine a machine screw size.