Screws provide a strong hold and an anchor for materials like wood, drywall, sheet metal, concrete, and more. Choosing the correct screw size for a job is important to ensure proper application and longevity. Incorrect selection can result in loosening over time and potentially damaging the material that is being fastened.
The simplest way to determine what screw size you need is by measuring the screw’s gauge, which indicates its external diameter. The gauge of a screw is usually listed in fractions, with each number representing a different fraction of an inch. Screws that have a major diameter less than 1/4″ are often labeled from #0 to #14, and those with a diameter of at least 1/4″ are usually labeled with the number 20 (which refers to the threads per inch).
Besides gauge, screw sizes are also determined by their head-bore and shank-hole diameters. Generally speaking, the larger these two numbers are, the bigger the screw is. In the US, screw sizes are measured according to the Unified Thread Standard (UTS), which divides threads into coarse and fine thread series, based on their diameter and pitch.
While the UTS replaced older standards such as the Whitworth and Sellers thread patterns, companies sometimes continue to use these terms to distinguish their products from metric screws. When buying hardware, check the description to make sure that it is in fact made to the UTS. If it is not, the screw is most likely made to an imperial size and will be denoted as such in the description, for example “10 x 60”. 3/8 inch to mm