Screws come in many different types and sizes to help with a wide range of construction tasks, from woodworking to metal roof installations. Choosing the right type and size of screw for a job is important to ensure a strong hold and structural integrity. As you shop for collated screws online, or sift through a drawer of random screws and bolts, understanding three basic measurements can help you find the right ones for your next project: screw diameter, threads per inch (TPI), and length.

Screw diameter refers to the outer threaded portion of a screw. It is commonly measured with a tape measure or screw gauge, but you can also use your caliper to get an accurate measurement. A screw’s diameter is typically the first number indicated in the screw size, and it corresponds to the table below. For instance, a #8 screw has a diameter of 5/32″, and is available in lengths ranging from 1/12 to 3″. This type of screw is best used for light-duty applications.

Depending on the material into which you are screwing, you may need to select a sturdier or finer screw. Fine applications generally require a screw with fewer threads, while sturdier materials may benefit from a screw with more threads. In either case, the screw’s length should be matched to the thickness of the materials you are joining together.

In the United States, most fasteners are sized according to their outside diameter and thread count. There are two standard thread standards in the inch series: coarse (UNC) and fine (UNF).

When selecting a screw, you should also consider its head-bore and shank-hole sizes. This information is usually included in the screw’s callout, which may also include a tolerance class, a left-hand symbol (LH), or a letter designation indicating the head style.

While screw size is an essential consideration, it’s not the only factor when choosing a screw for your project. It’s also important to consider the screw’s length, and how deep it needs to penetrate into the material. Choosing the incorrect screw length can split or weaken materials, and it could lead to the screw becoming loose over time.

Screws offer much more holding power than nails, and are often preferred for projects that will be exposed to heavy stress or vibration. Screws can also be driven into wood and other materials without drilling pilot holes, making them more versatile than nails in a wide range of projects. However, determining the correct screw size for your project can be confusing, as plans, instructions, and specs reference specific kinds and sizes of screws all the time. Lee Grindinger is here to clear up any confusion about what all those numbers and letters on a screw mean. metric to standard