Screws are an essential construction tool, used for a wide variety of tasks. But choosing the wrong type of screw can cause them to bend or break, and can damage the material they are being inserted into. It is important to select the right screw for the job at hand, but how do you know which one to pick when there are so many options? There are three key factors to consider when selecting a screw: the gauge, length and threads per inch.
The gauge of a screw is a measurement of its diameter, and determines the strength with which it can be anchored. Using the wrong gauge can cause the screw to slip or bend, and may not provide a strong enough grip. The screw’s gauge depends in part on the thickness of the material it is being inserted into, and also on the amount of weight it will support. For example, a heavier material will require a thicker screw of a higher gauge than a lighter material.
There are two common systems for measuring screw sizes: the imperial system and the metric system. The imperial system measures screw sizes in inches, while the metric system uses millimeters. Most hardware stores stock both systems, with some exclusively carrying imperial and others primarily selling metric fasteners. Screws are produced in a variety of thread types, and the type of thread you choose should match the material you’re screwing into, as well as your tools and your project needs.
Threads are created through a series of subtractive processes: tapping, drilling, milling, cutting, die casting, machining and grinding. Screws are manufactured to suit a number of different manufacturing standards, each of which has its own range of screw sizes. Some of these include the UTS system, the Whitworth system and a number of other proprietary systems. Despite the myriad of different system specifications, most hardware is made to standard ISO metric threads that are gradually displacing older inch-based standards.
Once you’ve established the screw’s diameter, you need to figure out its thread count or “pitch,” which is a measurement of how many threads are contained within an inch of the screw’s length. This is typically listed in a table along with the screw’s major diameter, and comes after the gauge. For example, a screw with a diameter of 5 mm and a pitch of 1 mm would be labeled “5.0 x 60.”
When you’re shopping for screws, look for a number listing the gauge and length first on the packaging. The threads per inch are usually listed between the two numbers, such as 6-32 x 1 1/2″, or are separated by a comma, like 10-35 x 2″. Occasionally, screws will have the thread count included in the diameter measurement, such as 10-35 x 1.75″, which would mean that the screw has a thread count of 35 and is a length of 1.75 inches. This is very rare, however. 3/4 to mm