When woodworking plans, instructions or specs reference specific kinds and sizes of screws it is important to know how the measurements are expressed. This is especially true with the imperial system and its variations, and the metric system.

Screws that don’t take a mating part are often denominated with Industry Numeric Sizes that don’t show the number of threads per inch designation.
Length

The length of a screw is important because it determines how far into the material it will go. If a screw is too short, it won’t be able to grip the material and hold it securely. If a screw is too long, it may protrude out of the material and cause damage or pose safety risks.

Screws are typically sized by their major diameter and their length. The major diameter is the largest dimension of a screw’s thread, measured from the crest or top of one side to the crest on the other side. This measurement is also referred to as the gauge size, and it’s usually rounded to simplify reference and avoid unwieldy decimals.

The length of a screw is also important because it affects the screw’s strength. In fact, multiple studies have shown that equivalent stress decreases with increasing screw length and diameter. Considering all of these factors when choosing a screw size is vital to ensure a successful and reliable connection.
Diameter

Choosing the correct screw diameter is important for a secure hold and proper application. For this reason, Accu recommends using precise tooling such as a Vernier Caliper to ensure accurate readings.

The diameter of a screw thread is measured along the peaks or “crests” of the threads. This measurement is known as the major diameter and is often indicated by the first number in the thread size designation.

The root or base of a thread is the area between the crests. This dimension is known as the minor diameter. The root diameter is important because it affects how much stress a screw can withstand when it’s engaged in bone. Studies have shown that larger screws can withstand more load than smaller screws. However, one study found that larger screw sizes did not improve fixation in osteoporotic spines compared to small-diameter screws. The researchers suggested that the difference in outcomes could be due to a combination of factors, including insufficient sample size, wide individual variation in bone quality and level tested, and mismatched screw properties.
Thread Pitch

Screws are used for a myriad of construction tasks, and getting them wrong can split wood or affect the soundness of a structure. Choosing the right screw length and width is important, but even more crucial is ensuring that you have the correct thread pitch. Thread pitch is a measurement of the distance between adjacent crests (or peaks) in the helical thread. It is a bit more complicated than measuring the diameter of a screw, but it can be done using either a pitch gauge or a caliper.

The thread pitch determines how far the screw will travel after a complete revolution. The higher the thread pitch, the farther the screw will travel. It is also a factor in determining the lead of a screw. Lead describes the distance between two successive thread crests, and is easier to measure than pitch. It is measured by taking the difference in pitch between two adjacent crests and multiplying it by the total number of thread crests on the screw.
Material

The material used to make a screw affects its performance. For example, it can be easier to drive a short screw into a thicker material than a long one.

Screws are made from a variety of materials, including brass, steel, stainless steel, and polymers. Different types are intended for different uses and have varying properties.

Many systems of identifying screw sizes have been used, but the ISO metric preferred series has largely displaced other standards. Other commonly used systems include the Whitworth, British Standard, and BA system (British Association).

In inches, screw size is typically indicated by the gauge first, followed by the thread count and shaft length. Screws with heads that sit above the material, like wood screws and drywall screws, are usually identified by the head type. Other types, such as lag screws and machine screws, are identified by their diameter, which is often measured in millimeters. This information can be found on a product’s packaging or in a table.1/4 lag screw pilot hole