Screws come in a variety of sizes, shapes and heads. They also have varying thread patterns to fit the materials they are intended for. Choosing the right type of screw for your project helps increase the strength and longevity of the fastener. It can also help ensure that the fastener will properly anchor and hold the material. Knowing what size screw is required for a specific job can help make sourcing and using them much faster.
The size of a screw is determined by its gauge, threads per inch and shaft length in inches. The first number is the gauge, which refers to the screw’s outer thread diameter or major diameter. The second number indicates the screw’s pitch, or distance from one thread peak to the next. Screw threads are usually measured in a standard system known as Unified Thread Standards (UTS) or in the more widely used metric system of measurement.
For UTS screws, the first number on the packaging indicates the major diameter. The second number indicates the threads per inch. The higher the number, the finer and tighter the threads. The metric system of measuring threads uses a different method to determine size, counting the number of thread peaks in an inch.
Generally speaking, a screw should enter the material at least half its thickness to ensure proper anchoring. If the screw is too short, it will not have sufficient shear or tensile strength to hold the materials together. Likewise, a screw that is too long could damage the opposite side of the material. screw size chart