If you have a bunch of screws lying around the house and need more, or are looking for a new type of screw to use, knowing how to read screw sizes will help you make an informed purchase. The key factors to keep in mind are the screw’s diameter or gauge, the number of threads per inch, and the shaft length. When shopping for wood screws, a general rule of thumb is to select a screw that will enter the bottom board by about half its thickness.

Screws are usually marked with a number, a letter, and a number of additional characters to indicate their size and type. For instance, a screw with the numbers 6-32 is a wood screw that has a 32-tooth thread count (almost double the normal number of threads on standard wood screws) and an inch and a half of shaft length, not including the head. The diameter, or gauge, is indicated by the first number on a screw’s callout, which usually appears at the top of the package. Screws with a larger gauge have thicker walls, and are typically used for heavy or thick materials, whereas thinner gauges have finer threads and are better suited to fine woodworking projects.

The next number on the callout indicates the number of threads per inch. Screws have a number of different thread counts, and this is what differentiates one type from another. The higher the thread count, the tighter the screw will be. Typical screw threads fall under the category of National Coarse or National Fine, but other types are available.

Once you’ve figured out the threads per inch, you’ll need to know the screw’s shaft length to determine whether it will be suitable for your project. The shaft is the section that extends from the screw’s head, and it can be threaded or unthreaded. Screws with an unthreaded shaft are called drywall screws, while screw with a threaded shaft and a raised head are called finish screws.

Using the information in this article, you can figure out how much of a screw’s length is buried by measuring from the end of the shank to the point where it begins to taper. The head diameter is about twice the shank diameter, and it can be determined by multiplying the screw’s major diameter by 0.013 in and subtracting 0.006 in.

A note about imperial measurements: Below 1/8″, imperial screw sizes are numbered rather than measured, starting with #12 and increasing by adding zeroes, reaching #0000, which is super tiny! Metric screw threads are based on the same principle, but with a few additional rules. The first number on a metric screw’s callout will always indicate the diameter, followed by the pitch and then the shaft length in inches, not including the head. For example, a screw with the callout M6 x 1 is 6 mm in diameter and 1.5 mm in pitch. The diameter is measured in millimeters, while the pitch is expressed as a decimal with the value less than 1. #6 screw diameter

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