Screws are a staple in any toolbox, providing a versatile means to fasten and secure objects. They also come in a wide variety of shapes, head types and lengths, making them useful for everything from product design to construction projects. Knowing the proper screw sizing for your specific needs will help ensure the best possible fit and increase the longevity of each screw.

Screw sizing is determined by the major diameter of the threaded portion of the screw, known as the gauge measurement. It is then further defined by the number of threads per inch, or TPI, which can be measured with a simple caliper. Simply measure the number of peaks along a one-inch section of the screw and divide that by the major diameter, to determine the screw size.

The imperial system measures screw sizes by a gauge number that corresponds to fractions of an inch. In order to calculate a screw’s diameter, simply match up the gauge number with a corresponding fraction of an inch on a screw gauge chart like this one from Engineering Toolbox.

The metric system uses millimeters to describe screw thread sizes, so measuring them requires a caliper as well as understanding the basic concepts of the metric system. Screws sold with metric measurements usually list the diameter first, then the length in millimeters next, followed by the thread count in mm (a screw labeled as 5.0 x 60, for example, has a 5 mm diameter and 60 mm length). Screws also come with different types of heads, including slotted, Phillips, square and Robinson, that are used to drive or screw into a variety of materials and surfaces. 5/16 inch to mm