How to Identify Screw Sizes

Whether you’re sorting out an organized drawer of random screws or shopping for a few dozen to complete a project, it’s important to know how to properly identify screw sizes. The three essential measurements to keep in mind are gauge, length and thread per inch (TPI). This will help you match the right screw for the job at hand.

Screws come in a huge selection of types and sizes to meet the requirements of virtually any task. Choosing the correct screw for the job is crucial for ensuring proper performance and safety. For instance, using the wrong screw size can split wood or compromise the structural integrity of a building. Incorrectly sized fasteners can also cause unnecessary strain, which can lead to premature failure and damage.

There are two main systems of screw thread measurement: the Unified Thread Standard, which is used in the United States, and the metric system, which is used worldwide. Both systems are similar, but differ in their methods for determining screw diameter and pitch.

For example, to determine the gauge of a screw in the Unified Thread Standard, you measure its major diameter and divide it by the number of threads per inch. For example, a screw with a major diameter of 12 inch is a 6 gauge. In the metric system, you use a caliper to measure the distance between adjacent threads at their peaks.

Once you’ve determined the screw’s major diameter and threads per inch, you can find out its corresponding sizing in inches by reference to a handy table. The equivalant dimensions in both inches (fractions & decimals) and millimeters are shown in the chart below, which lists the equivalant measurements for a number of different screw and bolt sizes. 5/8 to mm