Screws come in a huge range of sizes, so choosing the right one for your project can be challenging. The correct screw size is important to ensure that the screw will fit into the hole you are drilling, and can hold the weight of whatever you’re fastening. To determine the correct screw size, you need to know a few things: the head-bore and shank-hole diameters; the screw’s gauge; the thread pitch; and the length of the screw.
Most wood screws are sized by their diameter, or “gauge,” rather than by their length. A gage number is listed on the package, along with a letter indicating the type of screw. For example, a “6” screw means it has a 6 inch head-bore and shank hole and a 6-32 thread pitch. Screws are also numbered by their thread count, which is the number of threads per inch on the screw. The higher the number, the finer the thread; the lower the number, the coarser the thread.
In the US, screw sizes are often designated by the USS (United States Standard) or SAE (Society of Automotive Engineers) series. Both of these are based on the earlier Joseph Whitworth thread design, which was augmented by various national and inter-company standards. In the late 19th and early 20th centuries, as Europe leaned toward the ISO metric system, these national standards were superseded by a unified international metric screw thread standard called Sellers.
Screws are also sized in the metric system, and are referred to by their diameter and length. Metric screw measurements are usually given with the diameter first, followed by the length in millimeters. For example, a screw labeled “5.0 x 2” means that it has a 5.0 mm diameter and is two inches long. 1/4 to mm