Screws come in a wide variety of sizes to handle tasks both large and small. Choosing the right screw size is vital to ensure compatibility between the screw and the threaded hole in which it will be installed. Screw incompatibility is often caused by mismatched thread diameter, thread pitch, or length. Fortunately, there are three basic measurements that can help you determine the correct screw for any given application.
When shopping for screws, look for the callout on the packaging to find out the key numbers you need to know to choose the correct screw for your project. The first number is the screw gauge, or major diameter. This measurement is easy to convert to decimal form using a handy chart like the one at Engineering Toolbox. Simply match up the gauge number with a fraction of an inch and you’ll have the equivalant screw diameter.
The second number on the screw callout is the thread pitch. This is the distance between two adjacent thread peaks on a one-inch section of the screw. This measurement is not as easy to measure, and you’ll need a caliper to get a precise reading. However, most screw manufacturers use a standard thread pitch that corresponds to each major diameter.
The third number on a screw callout is the length of the screw shaft from its head. This is usually listed in millimeters, but it’s a good idea to check the metric system table below to see the corresponding imperial measurement. The shaft length also indicates whether the screw has a coarse or fine thread, and if so, which class. screw size chart