Screws are a staple tool for almost any construction project, but choosing the wrong size can split wood or affect the soundness of a structure. Understanding the three essential measurements of a screw is vital for any contractor: gauge, length and threads per inch (TPI). This article covers those measurements and how to use them to select the correct screws for your project.
Screw sizing is usually listed with the gauge, length and thread count on the package. The gauge is the external diameter of the screw, and it ranges from a size #0 to #12 in the imperial system. The length is the distance from the head to the shank in inches, and it ranges from a short, stubby screw to a long, fat wood screw.
The threads on a screw are the spiral pattern that runs around its cylinder, and they can be tapered or parallel, full or partial. Threads also come in different pitches, or threads per inch. Choosing the right thread pitch is important, because it determines how much shear strength a screw has.
During World War II, issues with interchangeability of inch-based fastener standards led to the creation of the Unified Thread Standard (UTS), which separated coarse and fine threads into two distinct series. The lower or coarser thread counts are described as Unified Coarse Thread (UNC), and the finer threads are known as Unified Fine Thread (UNF). The different pitches are numbered accordingly, and there is a conversion table available that converts the gauge number to the thread count. 2 in to mm