When working with screws, knowing how to accurately measure length and diameter can save you time and effort. It’s a critical skill set for DIY enthusiasts, mechanics, and professionals. It can also impact the structural integrity, resource allocation, and professional reputation of any project. In this article, we’ll take a look at how screw sizes are measured and what the different measurements mean in terms of performance and application.

Screws are commonly labeled with three dimensions: gauge, threads per inch (TPI), and shaft length in inches. You can use a ruler or tape measure to determine the shaft length, while the major and minor diameters can be determined by measuring from the crest of one side of the thread to the crest on the other side. The numbering system gets a bit trickier with smaller imperial screws, where the naming starts with a leading zero and then increments as the size increases.

The gauge, which is typically a decimal number, defines the outside diameter of the screw’s threads and corresponds to a specific thread pitch. To find the thread count, simply count the number of thread peaks over a one-inch length. This measurement is then combined with the major diameter to define the screw’s overall size. Engineering Toolbox has a handy chart that lists screw gauges and their decimal equivalents.

The head size refers to the dimensions of the screw’s head, which can be flat, pan, hex, or button-shaped. The head shape can influence the strength and holding power of the screw, as well as aesthetic considerations. For example, a flat head screw is ideal for drywall while hex or square drive heads are more suitable for applications that require higher torque.    5/16 lag bolt pilot hole