Choosing the right screw size for a construction project can be a tricky task. Screws come in a wide variety of lengths and gauges for an endless list of applications, but choosing the incorrect one can split wood or damage the soundness of a structure. The correct screw length and gauge will depend in part on the width of the material into which you are screwing as well as how much weight the screws will support. The correct gauge will also depend on the type of wood or other material you are screwing, as different materials call for different types of threads.

Screw sizes are usually described by three numbers – the gauge, threads per inch (TPI), and length either in inches or millimeters. Screws are normally labeled with the gauge first, followed by the TPI and finally the length. This will allow you to quickly identify the correct screw for your job.

Gauge
Screw gauges range from 2 to 16 with 8-gauge screws being a good general-purpose size. Your selection will also depend on the width of the material into which you’re screwing, as heavier materials require thicker screws with wider gaps between threads to support their weight.

Threads Per Inch (TPI)
Screw threads are measured in the UTS system, which specifies a number of different thread series. The lower or coarser thread counts are specified as Unified Coarse Thread (UNC), while the finer thread counts are specified as Unified Fine Thread (UNF). To determine a screw’s thread count, simply count the number of thread peaks along a one-inch length of the screw. 3/16 to decimal