Screws are used in a wide range of construction tasks, from woodworking to metal roof installations. Screws are stronger than nails and provide a more secure connection that will not come loose over time. The right screw size will also minimize the amount of force required to drive the screw into the material. Screw sizes are specified by three numbers: the gauge (or diameter), thread count, and shaft length in inches.
Screws have a variety of thread types and pitches, designed for different purposes. For example, wood screws have coarser threads, while screws for metal roofing require finer threads to resist damage. In addition to the thread type and pitch, other factors such as the material of the screw and its intended use must be considered when selecting a screw size.
While determining screw size can be difficult, there are several ways to make the process easier. Typically, the first number on the screw package will indicate the gauge. For example, a package of screw that lists its measurements as “6 x 32” indicates that the screw has a #6 gauge and is an inch and a half long.
Screws sold with metric system measurements on the packaging will usually list the diameter and length in mm, followed by the threads per meter. The threads are produced by a subtractive process that can include various kinds of cutting (single-pointing, taps and dies, milling); molding; casting; forming and rolling; grinding; and lapping to follow. 3.5 mm to inches