Screws come in a variety of shapes and sizes, and they can also have different types of thread patterns that are used for specific applications. Using the correct type of screws helps ensure that your project is completed properly and can hold up to regular use and abuse. Screws also have varying lengths and heads that are designed to fit with different power tools or attachment points on power drills. The type of screw used can also impact how fast and easily it is to complete a project.
In the United States, screws are usually sized by their diameter and threads per inch. This is because of the Unified Thread Standard (UTS) that uses an inch-based system. There is a metric system of screw size measurement as well, but it has not yet gained much traction in the United States.
The first number in the screw size is the screw diameter in inches, and the second number is the screw threads per inch. This second number may also be listed as a decimal or fraction depending on the size of the screw. A hyphen is typically added between the decimal or fraction and the threads per inch to distinguish between coarse and fine screw threads. In either case, a larger number indicates a larger screw diameter and a smaller number means a finer thread.
When choosing a screw for a project, the three most important characteristics to consider are the screw gauge, length and material on which it can be used. Selecting a screw that is too thick can damage the material and lead to premature failure, while a screw that is too short may not anchor properly.
For example, a #5 screw is 7/64 inch in diameter and is available in lengths from 3/8 to 3/4 inches. This small screw is best suited for light-duty projects, such as crafts and children’s toys. A #8 screw is 5/32 inches in diameter and is available in lengths from 5/8 to 3 inches. This medium-duty screw is ideal for cabinetmaking and other general construction.
Screws that are rated for heavy materials and can hold more weight require a larger gauge. For instance, a 10 gauge screw can be used for wood, while a 12 gauge screw is suitable for concrete or sheet metal.
In the metric system, the screw size is determined by measuring the diameter of the head and the pitch. A screw’s diameter is measured in millimeters and the pitch is measured by counting the number of thread peaks on a one-inch length. There are several ways to measure the pitch, but the most accurate method is to use a calibrated tool. The following tables provide a convenient reference for the metric conversion of the most common screw sizes. 1/4 to mm