Screws come in a wide range of shapes and sizes for use with different materials like wood, drywall, sheet metal and concrete. Using the proper size helps ensure long-term strength. But choosing the wrong size can split wood or damage a piece of equipment.
The size of a screw is usually listed with three important numbers: the gauge (also known as the diameter), the threads per inch (if used with a nut or in a threaded hole) and the length in inches. The numbering system for screws is the Unified Thread Standard (UTS). Screws are sized by their major diameter, also called the “gauge.” The gauge is designated by a letter with increasing values: A, B, C, D, E, F, G and H. Engineering Toolbox has a helpful chart that converts each of these to decimal equivalents, so you can easily determine what size screw you have.
When determining the screw gauge, you need to take into consideration what material you’re installing the screw in and how much weight it will support. Thicker and heavier materials require higher gauge measurements. Conversely, fine pieces benefit from a lower gauge.
When looking at the head of a screw, you can determine the screw gauge by measuring across the top of the head where the oval countersunk portion and the rounded head portion meet. This is the point where you will start counting threads, and will find that the screw with a smaller gauge will have more threads than the one with a larger gauge. 2 in to mm