Screws are the staple of construction tasks, from fixing a shelf to installing metal roofing. A wrong size screw can split wood or damage the structure of a project. Understanding the three fundamentals of screw sizing — gauge, length and threads per inch — helps you select the right screw for any task.
Screw gauges are the first numbers listed on a screw, and they indicate the diameter of the head or shaft in standard fractions of an inch. Screws with smaller heads have a smaller gauge number, while those with larger heads have a higher gauge number. The number is followed by the threads per inch, or TPI, which indicates the amount of space between thread peaks in one-inch of a screw. Screws with coarse threads have a bigger TPI than those with fine threads.
The next number on a screw is the length of the screw in inches, also in standard fractions of an inch. The most common screws are 2 through 10, although some projects require a different size. Screws in the imperial system usually list the gauge first and the length second, with the TPI included between them, like 10-35 x 2. Screws in the metric system use the opposite order of numbers, with the diameter (also known as major diameter) of the screw’s head or shaft listed first, followed by the length in millimeters, or mm.
A screw’s head type can have a significant impact on the amount of force required to drive it into place, which in turn affects how long it holds. The most popular head types are flat countersunk heads, oval countersunk heads, round heads and pan heads, which are pictured below in Figure 1. Each type of head has a specific point where it rests on a surface when installed.
Screws have a variety of different thread types, depending on what they will be used for. Drywall screws, for example, are available with coarse or fine threads to prevent tearing when they’re driven into drywall. Wood screws have coarser threads than drywall screws and are sometimes available with an unthreaded portion near the head, which helps protect the wood.
When choosing a screw for any project, make sure the length is longer than the material thickness, and don’t go much longer than that, or you risk damaging the material. If you’re uncertain about the correct length for a screw, try to select a length approximately half of the material’s thickness. Going much longer than that will result in a screw that does not anchor properly and could potentially damage the opposing side of the material. The quickest way to choose the correct screw is to look at a screw sizing chart or consult a sizing guide. Most screw boxes will have a chart with all the information you need, or you can check online for a comprehensive screw sizing guide. 5/8 to mm