Screws are used for a wide variety of applications and come in many different shapes, materials and sizes. The right kind of screw for the job will ensure that it is secured tightly and will stay in place. Screws also come in different head types, such as slotted, Phillips, square and Robinson, as well as threaded shafts that are either coarse or fine for wood screws or metal threads for other fasteners.
Screw sizes are identified by a combination of two numbers: the gauge and the number of threads per inch. The first number is the gauge, which measures the outside diameter of the screw’s thread and is typically labeled from #0 to #14. Engineering Toolbox has a handy chart that explains screw gauges in inches and decimal equivalents.
The second number is the threads per inch, or TPI, which is measured by counting the number of thread peaks that appear on a one-inch length of the screw’s thread. Screw threads are available in coarse or fine varieties and may have a tapered or straight configuration. Screws with tapered threads are sometimes called wood screws, while those with straight threads are called machine screws.
For machine screws and other fasteners with a threaded shaft, it’s important to match the screw thread with the thread of the material you’re screwing into. If the threads are too large for the material, they’ll loosen or bend and could break off. Likewise, if the threads are too small, the screw will not be secure and will rip out of the hole. 5/16 lag bolt pilot hole