Screw Sizes and Types

Screws are available in a wide range of sizes and types for fastening materials ranging from wood to metal. They are classified by things like head type, drive types, diameter, length and threading, among other factors. Choosing the right screw for a job depends on many things including the thickness and density of the material, the size and weight of the load they need to hold and the risk of them pulling out. A basic understanding of the different types of screws, their diameter and length, as well as their threading, will help you choose the best screw for any project.

Metric screw measurements are listed by the diameter first followed by the thread pitch (the space between threads) and then the length in inches. For example, a screw labeled M6 x 1 mm has a diameter of 6 mm and a pitch of 1 mm. Screws labeled with the imperial system usually have a gauge number (or major diameter) followed by the length in inches, for example, 3/8″ x 1 1/2″.

Length is important because you want the screw to enter and be supported by the material being attached. For binding, button, fillister, flange and hex heads as well as pan, socket and low socket, square and truss heads the length is measured from under the head. For bugle, flat and oval heads the length is measured from where the head rests on the material’s surface. For threaded rods and bolts the length is normally described by its diameter and number of threads per inch, for example, 1/8 x 24 threaded rod has a diameter of 8 mm and 12 threads per inch. pilot hole for 5/16 lag screw