If you’re building a piece of furniture or fixing a door, you need to know what kind of screws are best. Choosing the right screws depends on what kind of material you’re screwing into and how much weight the screws need to support.
Often plans, instructions or specs will refer to screw sizes in terms of major diameter and threads per inch. Here’s how to calculate those measurements and get the right fastener for your job.
Diameter
Screws are used for a wide variety of tasks in construction and woodworking, but choosing the wrong size can ruin a project. To avoid this, it is important to understand the three main measurements that determine a screw’s gauge, length and diameter.
The first number on a screw is the diameter, which is measured in millimeters. This is the larger number on a screw, and it can be found on a chart or by using a caliper to measure the distance from one peak of the thread to the next.
The second number is the length of the screw, which can be measured with a ruler or by using a go/no-go gauge. It is important to know the screw’s length because studies have shown that screw diameter and length are significant independent factors that influence bone-screw interface loads. Screws of smaller diameter were associated with higher load-carrying capacities and lower bone-screw failure rates. However, larger-diameter screws did not improve fixation of osteoporotic vertebrae in one study.
Length
While diameter and threads are fairly easy to understand, determining the correct screw length is sometimes a bit more complicated. This is because a screw’s nominal length can include or exclude the head, and the actual head-bore size is dependent on whether it is countersunk or not.
This is why some screws may have a different length for the same diameter, depending on their application. For example, using a shorter screw when attaching across the grain of the wood requires fewer threads and is less likely to strip.
The length of a screw is often listed on the package with two numbers and a fraction. The first number is the nominal length, which usually includes the head, while the second number is the threads per inch (TPI) count and will indicate a coarse or fine thread. A screw with a coarse thread should be used in wood that has been planed and sanded, while a fine thread should be used in wood that has not been planed or sanded.
Threads
The number of threads per inch – which is also often referred to as “pitch” – is found on the screw gauge that’s printed right after the diameter. In general, the higher the number is, the coarser the screw.
Screws with heads that sink completely into the surface (flat-head screws, for instance) are measured differently than regular screw sizes and are denominated using industry Numeric Sizes preceded by a # sign. These don’t include a threads-per-inch designation.
In the early 1860s, Joseph Whitworth developed a standard thread form that became widespread in the United States due to its use by major railroad corporations. It was called the National Coarse Thread, and it eventually led to myriad intra- and inter-company standards. Then in 1947, the ISO was formed and metric measurement systems began to supplant inch-based ones. The ISO metric system uses a different thread profile and naming conventions than the former USS, but it was quickly adopted worldwide and largely displaced the former standard.
Material
The material that screws are made from is important because it can determine the screw’s strength and resistance to corrosion. Some screw materials are also designed for specific uses, such as wood screws with rounded heads that reduce the risk of countersinking into wood.
Screws are typically made from materials such as carbon steel, stainless steel or hardened zinc. They are then threaded and plated with a coating to protect them from corrosion or provide a decorative finish (e.g., bright zinc or black oxide).
The type of material that a screw is used with dictates the gauge and length of screw required for a particular application. For example, sturdier materials require longer screws with a larger diameter to support more weight. Screws with shorter shafts are better suited for thinner materials. Screws also come in a variety of forms, from driver types such as Phillips, square and slotted to head shapes like hex, button, truss and round.1/4 to mm
