Screws are a common fixture in the workshop and can be used for a variety of applications. When you’re selecting a fastener for a specific project, knowing the right details about its size can save you from buying something that won’t work or drilling holes that are too big.
There are two important measurements to consider when determining the proper screw size: gauge and threads per inch (TPI). In imperial systems, these figures are typically listed on the packaging. The gauge figure corresponds to the diameter of the outer thread, and the length is measured either in inches or millimeters (mm).
The TPI measurement refers to the distance between one thread crest and the adjacent thread trough. Screws with fewer TPI have coarser threads, while those with more TPI have finer threads. You can measure the threads of a screw by counting the number of thread peaks along a one-inch length of the screw.
Once you know the threads per inch, you can determine the gauge by comparing it to a chart. For example, a #4 screw has 20 TPI, while a #10 screw has 35 TPI. When choosing a screw for a particular application, the length of the head should also be taken into consideration. For countersunk screws with rounded heads, you can use the point where the oval top rests on the surface to determine this. For non-countersunk round-headed screws, you can use a ruler or tape measure to measure from the tip of the head. pilot hole for 3/8 lag screw