The intricacies and specs of screw sizes can be confusing. In order to select the right screw for a project or to sort out a drawer full of loose screws, you’ll need to know the basic measurements. This article covers the basics of screw sizing and how to determine what a particular number means, whether you’re using an imperial or metric system.
Screws are measured in three ways: diameter, threads per inch, and shaft length. The first number is known as the gauge, and it’s a rough measure of the screw’s overall size. The second number is the threads per inch, and it’s a rough measurement of the spacing between adjacent threads on the screw. The third number is the shaft length in inches, and it’s a rough measure for how long you can screw into a material. Screws are often labeled with their gauge, threads per inch, and shaft length on the packaging or in a sizing chart. For example, a #6 wood screw has a diameter of 5/32 inches, has 32 threads per inch, and is an inch and a half long.
Woodworkers use a variety of different kinds and sizes of screws for a wide range of projects. Each type of screw has its own characteristics, such as the head shape and size, head length, shaft length, and diameter. Woodworking experts recommend choosing a screw with a head that fits snugly over the material you’re screwing into. This will prevent the screw from falling out or getting lost in the material. The screw’s length is also important, and the general rule of thumb is that it should penetrate at least half the thickness of the material you’re working with.
Depending on your project, you may need a special type of screw, such as an insulated screw. Insulated screws have a rubber coating that makes them less likely to damage delicate materials such as plastic or electrical wires.
Screws are also available in both a standard imperial system and the metric system, which is more common in other countries. Imperial-system screw dimensions are typically listed with the gauge and shaft length, while metric system screws are usually listed with their major diameter and head height.
Measuring a screw’s major diameter is relatively easy and can be done with a tape measure or ruler. Measuring its thread pitch can be a bit more tricky, however, as the measurements are very small and require precision. A good alternative is to use a thread gauge, which is a tool that contains strips of metal with various thread sizes cut into them. By systematically working through the gauge, you can match your screw’s thread size to an existing nut or drill hole. This can be especially helpful if you’re unsure of what the screw’s thread pitch is. Jaycon Systems engineers use these types of tools frequently to match screw sizes for our plastic parts and printed circuit boards. The information on these pages is provided for educational purposes only and should not be used as a substitute for professional advice. 3/4 to mm