If you’re a screw user, it’s important to understand how to read the measurements on your screws. Choosing the right screw size is crucial to ensuring that your fastener works properly with the material you’re working with, and can support the load required. It’s also vital to consider the length and width of the screw, and how these dimensions affect how the screw will perform in your application.
There are three basic screw measurements that are used to determine the appropriate screw size for an application: gauge, shaft length, and threads per inch (TPI). Understanding these dimensions will help you to quickly and accurately choose a screw for any job.
The first measurement is the screw gauge, which refers to the external diameter of the thread in inches. Screws with a major diameter less than a quarter inch are labeled with gauge sizes from #0 to #12, while screws with a larger major diameter will be labeled in fractions. Engineering Toolbox has a handy chart that lists screw gauges and their decimal equivalents.
The second measurement is the screw’s length. Screw length is important because it will determine how far the screw will be driven into the material. The general rule is that the screw should enter at least half the material’s thickness. Screws that are too short may break or split the material, while those that are too long will not provide adequate support and could potentially damage the material’s structure. 1/4 lag screw pilot hole