The world of screw sizes can be daunting for the amateur or professional fastener user. There are many different options for every project, and each has its own unique properties. Choosing the right screw for the job is crucial, and understanding the various dimensions will help you make the best choice.
Screws are typically categorized by the gauge size, which is the diameter of the external thread. For example, a screw with a gauge of #0 is thinner than a screw with a gauge of #12, and will hold up better in lighter materials. Screws are also rated by the number of threads per inch, or TPI. The higher the TPI, the more tightly the threads are formed, and the less friction there will be between the screw and the surrounding material.
A screw’s length is also important, and can be determined by its shaft size or head type. Screws with flat heads (like pan, socket and low socket) or hex heads measure their length from the bottom of the head, while screws with rounded or countersunk heads measure from under the head.
Generally, screw manufacturers will give the major diameter and threads per inch on the label, followed by the screw’s length in inches. However, in some cases the manufacturer will list only the gauge and shaft length, and leave out the threads per inch; for these you’ll need to use a standard thread chart or find online guides to determine the correct size. pilot hole for 3/8 lag screw