Whether you’re reading the callout on a box of screws or deciphering one that’s already been opened, understanding screw sizing is key. The first number on a screw label is the diameter; the second number refers to the threads per inch.

The correlation between the gauge number, head size, and diameter is complicated. However, there is a general rule that you can use.
Length

Screw lengths and gauges can be difficult to understand, even for experienced woodworkers. There are so many different types of screws and different variations on threads, that it’s no wonder that it can be confusing. Lee Grindinger, the editor of the Woodworker’s Journal, recently responded to a reader’s question about the meaning behind screw numbers and sizes.

He explained that a screw’s gauge size is not the same as its diameter. The reason is that imperial screw gauges are based on the diameter of the shank (the unthreaded part of the screw), while metric screw sizes use the head-bore size as their guide.

He also clarified that the screw length is measured from the bottom of the screw’s flat head to its tip. This is true for most screws, except those that can be driven entirely into a surface, such as truss-head, pan head or round-head screws. These are sometimes called countersinking screws. For those that can’t be driven completely into a surface, the screw length is given in addition to the head-bore and thread-pitch sizes.
Diameter

It can be confusing to read a screw size chart when you’re trying to figure out what kind of screws to use for your woodworking projects. There are a lot of different kinds of screws and each one has its own intricacies. In addition to the head size, the callout on a screw box may also include a tolerance class number, a left-handed symbol (LH), and the diameter of the threads.

Screw sizes are usually referred to in imperial units of measurement. The first number, known as the gauge size, refers to the diameter of the screw. The second number is the number of threads per inch.

The major diameter, or root diameter, of a screw is determined by dividing the gauge size by the number of threads per inch. For example, the major diameter of a #6 screw is 0.1380 inches. You can calculate the diameter of a screw with a calculator by multiplying the screw’s number by 0.013 and subtracting 0.060.
Threads per inch

Often, screws are described by their diameter and threads per inch, for example: 1/4″-20 x 2.” The first number refers to the screw’s diameter, and the second is its number of threads per inch. Threads are measured by counting the number of thread crests in a one-inch length. A machine shop uses a tool called a thread pitch gauge to measure the size of fasteners, but woodworkers can use a simple ruler.

Another important piece of information for a screw is its major diameter, which is the outermost area of the threads. This is also referred to as the root diameter, and it’s usually the same as the shank diameter or the smooth area of the fastener between the head and the threads. The Unified Thread Standard specifies a range of diameter/pitch combinations, known as a thread series. Coarse thread series are designed for general applications, while finer threads are more tolerant of adverse conditions and have higher tension strength.
Shank

The shank is the part of the screw that extends into the material. It is not threaded, and it’s used to hold the fastener in place. The shank diameter is sometimes referred to as the major diameter or root diameter.

The first number in the screw size indicates the major diameter, and the second number is the number of threads per inch. The major diameter and the threads per inch combine to give you the proper size screw for your project.

Wood screws are available in a wide variety of lengths and head types. Some are made of corrosion-resistant materials and are designed for outdoor use. Others are made of steel and are intended for precision woodworking projects.

It is recommended that you drill a pilot hole before driving wood screws into your project. This will help prevent the wood from splitting or breaking when the screw is driven into it. The pilot hole should be slightly bigger than the screw’s shank diameter.6 screw diameter