Screws may not seem like the most glamorous fasteners, but they play a very important role in all sorts of projects from wall framing to building wood benches. They hold everything together, and choosing the right screw for the job is essential to keeping your project looking professional. The screw aisle at your local hardware store is stocked with seemingly endless options. Depending on the type of construction or hobby project you are working on, different types of screws are needed. To make sure you are using the correct screw for your project, it is important to understand how screws are sized and what information is listed on a box’s callout.
The first number on a callout is the screw’s gauge, which indicates its diameter. The higher the screw’s gauge, the larger its diameter. The second number on a screw’s callout is the threads per inch (TPI), which is the number of threads that are in one inch of screw. The lower the screw’s TPI, the finer its threads are.
The third number on a screw’s callout, and sometimes the fourth or fifth, is the shaft length in inches. The longer the shaft, the farther into the material it will go. Screws with long shafts also tend to have more threads, as more threads mean that a greater surface area is engaged with the materials being fastened.
When you are selecting a screw for basic woodworking, it is generally recommended that the screw enters the bottom material by at least half of the thickness of the screw. For example, a screw with a #6 screw diameter would penetrate a 2 x 4 by 12.
The screw chart on our site shows the gauge, TPI and shaft length for each type of screw, but other information can also be found on the callout. For instance, if a screw has a special finish, you will see that noted on the callout, as well as its tolerance class, left hand symbol (LH), and whether it is a phillips or a flat head. #6 screw diameter