Screws are used to connect and fasten things in many kinds of projects. They offer a much stronger and more lasting hold than nails, which can loosen over time. They come in a variety of sizes and types, designed for specific applications. Knowing how to read a screw size label can help you select the right screw for a project. Screw sizes are listed by the diameter of the screw head, the threads, and the shaft length in inches. Screws are categorized by the Unified Thread Standard (UTS) or in the metric system, which is increasingly popular worldwide. Screw sizes are generally presented in a three-digit number format. The first number is the screw gauge, which specifies the diameter of the screw head. The second number is the number of threads per inch, and the third is the shaft length in inches. The first number is also called the major diameter, while the second number is known as the pitch. The shaft length is important, because it determines whether a screw will fit into a given hole or nut.
If a screw is specified as a coarse or fine thread, this will be noted in the first number of the screw size. A coarse screw will have more space between each thread, while a fine screw has less distance between threads. Screws can be classified as being left-handed, right-handed or neutral, depending on how the threads are arranged. A screw that is left-handed will have the letter L or R placed in the last position of the screw size, while screws that are right-handed have the letter N or D inserted in this position.
Screw heads may be smooth, hex, oval or round, and they can be countersunk or domed. The type of screw head you choose depends on your desired aesthetic or functional results, and may depend on what kind of tool you plan to use to drive the screw.
You may have heard that the head diameter of a screw is roughly double its shaft length, and this can be an easy way to remember what to look for on a screw label. In reality, the screw head diameter is more closely related to the shank diameter. The shaft is the long, thin section of a screw that extends from the head to the end where the screw will rest on a surface.
A screw size will also be indicated by a letter, symbol or number after the gauge and threads per inch. These values indicate what kind of material the screw is intended for, and also indicates how tight or loose the threads will be. Screws that have a tolerance class of 1 have the tightest threads, while those with a tolerance class of 5 have the loosest. 3/4 to mm