Screws come in a variety of lengths and diameters to accommodate different tasks. Choosing the right screw for a job is essential in order to get the work done and avoid splitting wood or compromising the integrity of a structure. Unfortunately, the numbering system used to describe screw threads can be confusing if you’re not familiar with it. Screws are sized by three different numbers: gauge, length and threads per inch (TPI). This article will explain the difference between imperial and metric systems of screw measurement and help you decipher screw callouts so that you can quickly determine which size screws you need for your next project.
Imperial Screw Measurement
Most wood screws are labelled with a two-part callout, which explains the diameter of the screw head and the length of the threads. The first number is the gauge, which is usually indicated by a letter followed by a decimal point. The second number is the thread count, which is the number of threads that fit in one inch. Screws with a coarser thread count are described as Unified Coarse Thread (UNC). Screws with a finer thread count are called Unified Fine Thread (UNF).
Screws that are labeled in the metric system use a three-part callout to describe their dimensions, starting with the major diameter of the screw, which is sometimes given in fractions of inches (e.g. M6 x 1mm). The last number is the tolerance class, which indicates how loose or tight a fit you need for your screw to be. screw size chart