Screws come in a wide variety of sizes. Knowing how to measure the diameter and length of a screw can help you purchase and use the correct screws for your projects. The article below provides a quick guide to measuring for screw size and some tips on the best type of screw for each application.
In the United States and Canada, screw sizes are based on a system called the American National Standard or UNC. The numbering system includes a gauge, threads per inch, and shaft length. The gauge number is usually one or two digits, and the threads per inch are described as fractions of an inch. The larger the number, the bigger the screw size.
The shaft length of a screw is measured from the flat bottom of the head (for round heads) to the tip. This measurement is often listed as the third number in a sizing chart, and it is followed by the “x” symbol for multiplication. For example, a 5/8-24 x 1″ screw has a shaft with a diameter of 5/8″, 24 threads per inch, and a length of 1″.
In the European Union and other countries that use the metric system, screw sizes are based on millimeters. Metric screw sizes are typically stepped up in 0.5mm increments. To determine a screw size in the metric system, you need a caliper or other means of accurate measurement. Engineering Toolbox has a helpful online screw and bolt scale that lists common gauges of US and metric screw sizes with their decimal equivalents. 3/8 lag bolt pilot hole