The Screw Sizes you select for your projects can have a huge impact on how the fastener performs. Incorrectly sized screws can cause the material to fail in shear strength, the amount of load perpendicular to the threaded section. Screw sizing is generally determined by gauge and length.

Gauge refers to the diameter of the external thread on a screw. Screws with a major diameter below a quarter inch are labeled from #0 to #12, while anything above that is labeled in fractional inches (for example, a 1/4" screw is labeled as 6). Screws can also be categorized by their thread pitch, which is the number of thread peaks on a one-inch segment of the screw. Screws with a higher pitch are finer, while those with a lower pitch have coarser threads.

A common sizing rule is that you should select a screw that is approximately half the thickness of the material it will be used in. A screw much longer than this will not anchor properly, while a screw shorter can cause the material to rip off or deform when the screw is pulled out.

Many screw types have different sizes that suit different purposes. For example, wood screws have a coarser thread with larger spaces between the threads than sheet metal screws, while drywall screws are available with coarse and fine threads to best match the screw to the material. In addition to gauge and threading, you may need to consider other factors such as the type of head on your screw, or whether it has a blunt or sharp point. 1/4 to mm