Screws come in a wide variety of sizes for a multitude of construction tasks. Knowing the three fundamental measurements when selecting screws will help you avoid frustrating projects – a screw too short won’t anchor properly; too long and it can damage the material on the opposite side of the wall.
Diameter
Screws come in all shapes and sizes for a wide variety of tasks, from woodworking to metal roofing. Choose the wrong size and you could damage the material or create a weak point that might not hold up to stress. But getting the right screw for an application starts with knowing how to identify the correct measurements.
When looking at a package of screws, the first number indicates the diameter and the second number is the length. Diameter is measured from the crest or peak of the threads to the trough or valley between them, and it can be expressed as a major diameter (also called a full diameter) or minor diameter.
A recent study using finite element (FE) analysis found that larger screw diameter improved fixation in osteoporotic vertebrae, but only immediately following implantation. However, this type of testing can be prone to biases such as differences in material properties, bone-screw contact area, and the experimental environment.
Length
When building woodworking projects, using the correct length screw is essential for a strong and secure connection. Screws that are too short won’t hold the material in place, while those that are too long can cause damage or protrude from the surface of the materials being fastened.
To make sizing screws easier for users, they are typically listed with three numbers – the gauge or diameter, the thread pitch (or reciprocal of the number of threads per unit length) and the screw’s nominal length in inches or millimeters. For example, a 10 x 2 inch screw means that the screw has a gauge of 2 and is 2 inches long.
In the US, imperial system screw measurements are commonly written with the gauge first and then the length followed by the threads per inch, while in other parts of the world, metric screw specifications are more common with the length listed before the gauge or threads per inch. See our comprehensive table below for the equivalant measurements in both systems.
Threads
A screw size is based on two things: diameter and number of threads. Different materials require varying types of screws, and it’s important to understand what type of screw you need for your project before selecting one. This will help ensure that the screw will be able to properly hold its intended weight and prevent damage or injury.
A great deal of engineering effort went into establishing standardized inch-based screw thread standards during the early 1900s. This included the creation of classes of fit and the development of new methods for producing and inspecting threads.
However, this wasn’t enough to achieve the level of inter-company interchangeability required for widespread use. With the onset of World War II, inch-based standards began to lose ground to the metric system that would eventually displace them entirely.
Thread Pitch
The thread pitch, which is the spacing between the thread crests (or flanks), affects how much the screw will grip the nut. When the threads of a screw and nut are properly matched, there are moderate non-negative clearances between their roots and crests.
As the pitch diameter of a thread decreases, acceptable manufacturing tolerances get smaller. This allows for more precise adjustments. Using a finer pitch also reduces the amount of stress placed on the thread, which can improve strength and reliability.
The most common thread types include UNC, which uses coarse threads with a fixed pitch for each diameter; and UNF, which is designed with fine threads of a specific pitch size. These are often stocked and available at your local hardware store or online. Other thread types aren’t as widely used and may require a special order. For example, a Sellers or American thread is a coarse type of thread that was used in the US and Canada before the Unified Thread system was adopted.1/4 in to mm