Screws are used to hold together many different materials, including wood, drywall, sheet metal and concrete. They come in a wide range of sizes and types to meet specific requirements for use on different construction tasks. Correct selection of the right screw can prevent problems that may arise from improper or incomplete attachments such as wood splitting, corrosion and the like. However, determining the right screw size can be tricky. It requires understanding three basic measurements: gauge, length and threads per inch (TPI).
The numbering system for screw sizes begins with the gauge, which is the diameter of the screw’s outer threads as compared to its overall width. The next number is the length of the screw, in inches. The final measurement is the screw’s thread pitch, which is the distance between adjacent screws. Screws with coarse thread pitches generally have smaller spaces between their threads than those with fine threads.
In addition to the UTS system, there is a metric screw thread standard that is gradually replacing the older inch-based standards throughout the world. Screws with a metric thread are designated by the letter M followed by the screw’s major diameter in mm and the screw’s thread pitch also in mm (Table 2).
When purchasing collated screws, the gauge, length and thread count are always listed on the packaging. However, the length can sometimes be difficult to read, as they are normally presented in fractions and not decimals. This can be frustrating for tradespeople who need to convert the screw numbers into decimal equivalents, and it can make them less easily understood by non-tradespeople. 3/16 to decimal