Screws are used in a variety of applications and materials. Choosing the correct screw size is important for proper installation and to ensure that the screw can securely hold the material being fastened. Screw sizes are often determined by the materials being worked with and the amount of weight the screws must support. For example, heavier materials may require thicker screws to prevent the fastener from coming loose over time.
Screw diameters are usually listed in three figures, the first number being the gauge (or major diameter) and the second being threads per inch (TPI) and the third number is the length, either by inches or metric millimeters. The numbering system varies between different types of screws, however most common wood screws use the imperial (inch) system and are marked with a gauge number followed by a number for threads per inch and then a shaft length in inches.
For wood screws, the gauge size (or major diameter) will usually be stamped or labeled on the head of the screw. This gauge is often referred to as the “screw size” in the industry and increases from #0 (as small as 1/4″ major diameter) to #12 (over 1 1/2″). Screws over 3 inches will increase by quarters while those under 3 will increase by half.
The metric system uses a similar method of sizing screws but has an added dimension of thread pitch, which refers to the distance between the individual threads on a screw. A more finely spaced thread is considered coarse while a tighter thread is fine. 15mm in inches