The proper selection of a screw size for any fastening application requires the consideration of several important dimensions and qualities. One of the most important is screw diameter, which must match the inner diameter of matching nuts and washers. This dimension, listed first in a screw size chart, is determined by the diameter of the shaft of the screw at its head, or, for countersunk heads, from the flat side of the screw underneath the head. The term 'diameter' is also used to refer to the thread's 'pitch line', an imaginary cylinder axially concentric with the thread's pitch axis and intersecting its flanks at equidistant points (or, equivalently, one-half of the screw's pitch distance).

Minor thread diameter lives up to its name in that it represents the smallest of the two extreme diameters delimiting the height of the thread profile: a cross-sectional view of a single thread in its pitch plane reveals a sharp V form that is topped by the crests of its flanks. This profile has moderate non-negative clearances between the roots and crests of opposing threads, and is generally matched to a corresponding nut thread profile.

As a result, the diameters of most metric screws are stepped in a number of equally large fractional increments that correspond to common screw sizes and nut thread sizes. Thus, when shopping for a particular screw size, it is important to consider both the minor and major diameters of the thread as well as its length from the head to its tip. #8 screw diameter