Screws are used to attach or fasten a wide variety of materials, from wood and metal to plastic. It's important to choose the right type of screws for a job, as the wrong type can damage or affect the integrity of a structure. The correct screw length, head-size and gauge can help prevent splits, work loosening and even structural failure.
Screw size is usually noted by three numbers on a label, which describe the screw's major diameter, thread pitch and shaft length. A typical sizing may read "#8 x 1-1/4", which translates as a #8 screw with a 1 1/4" shaft (a length suitable for most applications). Screws also come in different head-sizes and types, including flat, hex, drywall, button, truss and round heads. A screw's length is determined by the material it is being installed into and can be altered by using a countersink drill bit or a screwdriver with a different hex-head.
The screw's thread pitch describes the distance between each individual thread. A screw with a coarse thread has a wider pitch, while a fine-thread screw has a narrower pitch. Some screws, such as drywall and wood screws, are available in both coarse and fine-thread options.
Screw measurements are typically expressed in inches, though some manufacturers use metric measurements for all screw sizes. In either case, the first number on a screw or bolt's label represents its major diameter, while the second number is the number of threads per inch. The last number is sometimes omitted, but if it is present, it's often listed as "TPI," which indicates the normal thread count for the screw.5/16 inch to mm