Screws come in a variety of shapes and sizes. The type you need depends on the material you’re fastening and the task at hand. For example, wood screws have features that help prevent wood splinting and finish screws have smaller heads that sit below the surface for a clean look.
Length
The length of a screw is the distance from the flat part of the head (or blunt end for non-head screws) to the tip or end of the shank. This is the most important measurement for determining whether or not a screw will fit into the material you’re using it in. A screw that’s too long will stick out from the surface and can cause injury.
Some screws have a flat underside that helps them bite into the material. These are called countersinking screws. Others don’t, and are known as non-countersinking screws. You’ll find them in a wide variety of applications, including woodworking.
Screw callouts usually include three things: the screw gauge, the threads per inch, and the shaft length in inches. Some also specify a tolerance class, which indicates how tight or loose the screw fits into holes and nuts. Screws with a tolerance class of 1 fit looser than those with a class of 2. You may also see an LH symbol indicating left-handed screws right after the tolerance class number.
Diameter
When it comes to screw size, there are several different systems that can be used. The most common system in the United States is the unified thread standard, while many other countries use metric measurements. Our screw size chart has information for both types of measurements.
In general, a screw’s diameter is based on its shank diameter. The shank is the long section of the screw underneath the head, and it can be unthreaded, partially-threaded, or threaded. The head diameter is usually roughly double the shank diameter.
For example, a #10 wood screw has a shank diameter of about 3/16” and a head diameter of about 5/32”. When a screw’s callout is printed on its box, it may also include a tolerance class number, an LH (left-handed) symbol, and the shaft length in inches. This guide explains all of these specifications and helps you understand the details on a screw’s callout number. This is a must-have for anyone who uses screws regularly.
Threads per Inch
The threads on a screw are what allow it to grip into the surface you’re screwing into. Knowing the number of threads per inch (TPI) will help you choose a drill bit that’s right for the job. This information is typically included on the screw packaging, and it’s listed right after the gauge. For example, a #10 screw has 32 threads in an inch.
The screw threads are V-shaped and have two flanks that meet at the center of the thread axis. Each flank is equal in height to the pitch, which is the distance between the thread crests.
A screw’s size is measured by its diameter and pitch, with the latter usually given first. Screws with an imperial system gauge are characterized by the screw’s major diameter and the threads per inch, while metric screws are described by their diameter and the distance between the thread crests. In either case, the formula to calculate a screw’s diameter is simply the screw’s gauge number multiplied by the threads per inch.
Material
Wood screws are a type of screw used for connecting and fastening wooden materials. They have features like shanks that prevent wood splint, twin threads for effective drive, and tips that allow them to be driven into wood without pre-drilling. They also come with a variety of head types, from square to Phillips to quadrex, so you can find the right one for your project.
Screws are used in all kinds of projects, from furniture and home repairs to construction and more. To make sure you’re using the correct screw size for your project, you need to understand how to read a screw size chart.
The first number on a screw size chart is the diameter of the screw’s thread. The second number is the number of threads per inch, or TPI. This is specified right after the screw’s gauge. If the screw has coarse threads, it will have a larger TPI than if it has fine threads.#10 screw diameter