When screwing things together, it's important to have the right kind and size of screws. Choosing the wrong ones can split wood or damage structures. Screws are available in a variety of types and sizes for an endless number of construction tasks, from woodworking to metal roof installations. But if you're not familiar with how screw measurements work, the numbers on the package can be confusing. There are three main measurements you'll see: gauge, length and threads per inch.
When shopping for screws, the first number on the packaging indicates the screw's diameter, or gauge. Screws with a smaller major diameter are labeled with a lower number, while those with a larger one are marked with a higher number. The number increases by a fraction of an inch with each successive screw size, and it's also listed in decimal form for easy reference.
The second number indicates how many threads are in each inch of the screw's major diameter. Screws with coarse threads have a higher thread count, while fine ones have a lower one. Metric screw threads are measured in millimeters, and their thread pitch is much more difficult to measure than the diameter of a screw, as it involves very minute spacing. To figure out a screw's thread pitch, use a caliper to measure the distance between adjacent threads at their peaks.
Finally, the third number indicates the screw's shaft length in inches. Screws with a shorter shaft length are often called "self-tapping" screws because they don't require pre-drilling holes to secure them in place. Screws with a longer shaft length are commonly used for structural projects, such as building decks or stairways.
The length of a screw is determined by the depth it needs to penetrate into the material it's being fastened to. For example, when screwing into wood, it's best to select a length that allows the tip of the screw to enter at least half of the thickness of the bottom board. Screws that are too long may protrude from the surface of the wood, causing damage and making it hard to secure the screw with a nut or washer.
Screws are much stronger than nails, and they're designed to hold up to expansion and contraction of the materials they connect with over time. But the right size and type of screw is important, because a screw that's too large can loosen over time while a screw that's too small won't hold the material securely.
If you're not sure which screw size is right for your project, you can always consult a Screw Sizes Chart. These handy charts show the equivalant imperial measurement in fractions and decimals of the screw diameter, as well as the common metric measurements. The chart will also help you identify the thread standard, such as course (UNC) or fine (UNEF).5/16 inch to mm