Screw Sizes – A Basic Guide

When building projects that use screws, choosing the correct size can be difficult. There are many factors to consider such as the type of material being attached, the head-bore diameter of the screw and whether it is threaded all the way to its shank. Additionally the force being exerted – shear versus tensile – must also be considered as this will influence the correct screw size and how long it can remain in place. This article is a basic guide to help you choose the right screw size for your next project.

Screw sizes are based on the standard inch-based Unified Thread Standards (UTS) system and include different thread series such as coarse (UNC), fine (UNF), extra fine (UNEF) and extra special. The threads on the head of the screw are also measured in a different way which makes them more complex than nails.

Using a screw with the wrong length can cause the screw to break off in the material being attached or leave a gap. For this reason, it is important to select the right screw for your application.

In the US, screw sizes are commonly listed as three figures – the gauge, the threads per inch (TPI) and the length in inches or millimeters (mm). The gauge is the basic size of the thread. The higher the gauge number, the finer the thread. Thread counts usually follow the gauge number and in some cases may be presented as two numbers such as ‘#10 x 36’. 1/4 in to mm

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