When you’re working with wood or other materials, the right screw can draw them together and hold them securely in place. Whether you’re installing an interior door or assembling a Nintendo NES cartridge, getting the right screw is crucial. Screws are often sold with their dimensions listed on the packaging, and the information you need is easy to find if you know what to look for.
The first number on the package indicates the diameter of the screw. This is also referred to as the screw gauge and it’s important to note that screw diameters can vary even within the same type of screw. For example, a #2 screw may have a larger head than a #3 screw, and that can have an impact on how well it holds fasteners.
Screw size is typically determined by measuring the major diameter and thread pitch of a screw. Threads are grouped into different series, such as coarse or fine, and the number of threads that fit into an inch of one-inch length is known as the threads per inch (TPI). The major diameter is measured using a caliper; the thread pitch is much trickier to determine because of the minute distance between each individual thread – you can measure this with a micrometer.
Once you’ve determined the screw’s diameter and thread series, you can then use this information to determine which kind of screw is best for your project. For example, if you’re building a deck, you might want to use #10 screws, which are 3/16 inches in diameter and come in a variety of lengths. These are suitable for general construction purposes, but you might also choose a #12 screw for heavier-duty projects.
Screws are also categorized by their tolerance class, which is indicated by the letter A or B next to the screw number. This is the indicator of how loosely or tightly a screw fits into a nut or hole, with class 1 being the least tight and class 5 being the most loose. Finally, you might notice the symbol LH if the screw is left-handed. This means that it has threads that turn in reverse.
If the screw you’re looking for is not a standard UTS size, it will be labeled differently on its packaging. This is because the metric system uses millimeters to identify screw sizes, and this measurement is specified after the major diameter and threads per inch. If you’re unsure what this means, check out our wikiHow article on How to Use a Caliper. You’ll learn how to make this precise measurement and determine the screw’s metric size. Once you know the mm of the screw, you can then determine its diameter by multiplying the head diameter with the shaft diameter. This is the same process that you’d follow if you were identifying an imperial screw, as well. This way, you can be confident that your screw is the correct size and will fit in the intended holes. #5 screw diameter
