Screws are used for more tasks than most people realize, so it’s important to know what size screw you need. Choosing the right screws will make your project go much more smoothly, especially if you use the correct gauge for your application.
Screws are sized by both diameter and thread count. The diameter is measured across the bottom cavity of the threads.
Length
The length of the screw is important because it determines how far the fastener will penetrate into the material. An improperly long screw will not secure the material and may cause damage to the substrate. A short screw will not be able to fully sink into the substrate and will not provide maximum load bearing capacity.
The screw size is typically listed as three numbers – the gauge, the threads per inch (TPI), and the length of the shaft in inches or millimeters. Often the middle number is absent, so that the screw’s nominal length is implied. When purchasing screws with measurements on the packaging, note that the diameter is always listed first followed by the length.
Some fasteners are coarse-threaded while others have fine threads, which are suited for varying types of materials. Understanding these differences will help you select the appropriate screw for your project. For example, if you are working with metal, you will want to select a coarse-threaded screw for optimal performance.
Diameter
The diameter of a screw refers to the outermost surface of the thread. It’s important to choose the right screw size for the application, as a large enough head will allow you to tighten the screw without damaging the material. A small screw on the other hand, may not be able to penetrate the tissue or could cause the screw head to deform when inserted into the hole.
The second number is the thread pitch, which is the distance between adjacent thread peaks. This can be coarse or fine, depending on the needs of the application. Coarse threads are typically used in larger diameter screws, while fine threads are often used in smaller ones.
When choosing a screw size, it’s important to remember that metric system measurements are different from standard inch-based measurements. In order to properly identify the correct metric screw size, you’ll need to know about three key measurements: gauge, diameter and length.
Threads per Inch (TPI)
The number of threads within one inch of the screw’s length is known as the threads per inch (TPI). A thread is a uniform band of protruding material that helically rotates around a screw.
The TPI can be derived through hand calculations, automated calculators or standardized tables. TPI is also known as thread pitch or lead.
TPI plays a large role in a screw’s holding power. A screw with a high TPI has more threads, and therefore a greater contact area, which increases the holding power.
The TPI also determines the tolerance class for the thread. A higher TPI allows for greater dimensional variation, while a lower TPI has more strict tolerances. This allows for a wider range of materials to be used with the screw. However, it requires more skill and effort to produce a thread with a high TPI, and the tooling needed for that is more expensive. This is why most manufacturers choose to use a low TPI thread.
Gauge
Screw sizes are generally described by three figures. The first is the gauge, the second the threads per inch, and the third the length in either inches or millimeters (described with a M prefix for metric screws).
The gauge of a screw is determined by its outside diameter. This is also called the major diameter and is labeled from #0 to #14. Screws with a major diameter less than a quarter inch are considered small and are usually marked accordingly.
The minor diameter is the lower extreme of the thread’s height and can be measured using go/no-go gauges or by measuring it directly with a calibrated scale. It is calculated as the major diameter minus the pitch. Screws with a coarser thread count are typically called Unified Coarse Thread, while those with a finer thread are known as Unified Fine Thread. For each gauge number, there is a decimal equivalent, which can be found by multiplying the integer by 0.013 and adding 0.06. Engineering Toolbox has a handy chart that lists screw sizes and their decimal equivalents.2 in to mm