Screws are used for a variety of tasks in construction and come in a range of sizes. Choosing the wrong size screw can cause damage and create problems with the materials you’re working with. Understanding the three fundamental qualities of a screw—gauge, length and threads per inch (TPI)—will help you avoid misapplication.
The first number you see on a package of screws is its gauge, also known as its major diameter. Screws with a major diameter less than 1/4″ are labeled in fractions of an inch, while those with a larger diameter are marked in decimal equivalents. Engineering Toolbox has a handy chart that shows you the major diameter of each screw, as well as its corresponding decimal equivalent.
Depending on the material you’re screwing into, you may need coarse or fine threads. Screws with coarse threads have wider spaces between them than screws with fine threads, which are more suitable for delicate materials like drywall.
The next number on the packaging is the screw’s TPI, which measures the distance between two adjacent threads at their peak. This measurement can be more difficult to understand than the screw’s major diameter, as you need a tool, such as a caliper, to measure the thread pitch. You’ll want to choose a screw with a TPI that matches the material you’re working with. For example, if you’re screwing into wood, select a screw with a TPI of at least 6 or 7. 3/8 inch to mm