Screws are used extensively in home improvement and construction, and there are a wide variety of sizes available for different applications. Standard sizing is critical to prevent the use of improper screws, which can damage materials or cause other problems. Screw dimensions include driver type (square, Phillips or hex), length, shank diameter and threads per inch. The size of the screw’s head is also important, as it determines how much pressure can be applied to the fastener and what kinds of tools will be able to drive it into place.
Screws may be sized using the Unified Thread Standard (UTS) or the metric system, which is growing in popularity in the United States. The numbering system for UTS screws is based on the screw’s major diameter and its threads per inch. The threads per inch measurement is easy to calculate — it’s the distance from one peak of a screw’s thread to the next, measured along the length of the screw. Using a caliper makes the job even easier.
The screw’s major diameter is called its gauge size, and it’s labeled from 0 to 14 in increments of 1/4″ for woodworking screws. After #14 the gauge size is indicated in metric units, and the first number refers to the screw’s major diameter, while the second indicates the number of threads per inch.
Unlike nails, which can sometimes come loose over time, screws have a better chance of staying securely fastened to their materials. This is because they are designed to resist expansion and contraction in the materials to which they are attached, a feature that’s especially helpful when working with soft wood. In fact, screws can often be used to replace nails in a variety of projects.
Aside from the number of threads, there are many other things to consider when choosing a screw. For example, screw length is an important factor, and a general rule of thumb is to select a length that will allow the screw’s tip to penetrate the bottom board by about 2/3 of the thickness of that board. Screws are also sorted by type, with square and Phillips heads being great fits for power drills and flathead screws for hand tools. Drywall screws, which are a staple of many home improvement projects, are available in coarse- and fine-thread styles to accommodate varying wall thicknesses.
Screws are also categorized by the materials they can be used for, such as metal and plastic. For drywall, for instance, it’s generally recommended to use coarse-thread screws; for metal or other materials that require a more precise fit, fine-thread screws are preferred. Screws can be sorted by diameter, too, with the sizes 4 through 12 being the most common for home and workshop uses. Screws of smaller diameters are generally only used for light-duty applications, such as assembling children’s toys and crafts. Larger sizes are primarily used in commercial and industrial applications. For these larger jobs, it’s a good idea to consult an experienced tradesperson for advice. 1/4 to mm
