Screws come in a wide array of shapes, sizes, lengths and head types. As the use of screws in products has grown worldwide, standard sizing has been introduced to ensure that all fasteners are compatible with each other and can be used safely together. The primary features of a screw include the screw’s head type, length, shank diameter and threading size. The sizing of fasteners allows for the easy identification of the correct driver type and head size, as well as to provide the necessary strength for various applications.

Understanding what these values mean is important when selecting the right screw for a project. Screws are commonly described by their diameter, thread spacing and length in both metric and imperial units. This guide will explain how these values are determined and what each means in the context of wood screws.

Screw sizes are often designated by a gage number that indicates the screw’s diameter and thread spacing. For example, a screw sized “3/8"-16” has a screw diameter of 3/8 inch and a thread count of 16 threads per inch. These numbers are also used to determine if the screw can fit into a specific nut or threaded hole. For wood screws, the first number is the screw diameter and the second number is the screw length (excluding the head unless it's a flat countersunk head).

The thread size is measured by measuring the distance between the center of one screw thread to the center of the next thread. The measurement is then divided by the number of threads per inch to find the number of threads per centimeter. Metric screw thread sizes are normally indicated by a number preceded by the letter M, and are typically stepped up in 0.1 mm increments.

While screw length is often defined by the application, it is important to select a screw that will not extend past the opposite side of the material on which it is being used. Screws that extend too far will be difficult to remove and may cause damage to the surface of the material. For wood screws, it is also recommended that the screw be a little longer than half of the thickness of the material to which it is being attached. This is to help prevent the screw from pulling out or loosening over time, as a result of expansion and contraction of the material. Screws are strong and durable, in contrast to nails which can easily loosen or break over time. They are built to last and are long-lasting, making them a preferred alternative to nails in many woodworking projects. Screws are also much easier to drive than nails, requiring less force and preventing the potential for nail heads to split or pull out. This is a major advantage when working with delicate materials like wood. Screws are also very versatile, being able to be installed in a variety of surfaces and applications. Screws are also much quicker to install than nails and require fewer tools.1/4" to mm