Whether you’re installing wood trim, hanging a new shelf or putting up drywall, choosing the right screws for a job can make all the difference. Choose the wrong screw size and it could split a piece of wood or ruin the structural integrity of your project. But if you know how to read a screw sizes chart, the process of finding the perfect fastener becomes much easier.
There are three main measurements when it comes to screw sizing: gauge, length and threads per inch (TPI). The first number on a screw package indicates the screw gauge, with sizes ranging from 1 to 5. The second number on a screw package is the length of the screw. Some packages include the threads per inch (TPI), but that’s more commonly included on metric screw packaging, where it usually appears between the length and the major diameter of the screw.
Screws also use a variety of acronyms to denote added features, such as an S for self-tapping (these screws don’t require a pilot hole before drilling). Some packages may include the letter “L” or “R” after the screw length to indicate left-handed or right-handed screws. And some packages will include the words “Counterbore” or “Faceplate” to specify that the screw is intended for countersinking (into a pre-drilled hole) or faceplate mounting (outside of a hole).
Understanding Screw Measurements
The first number on a screw package is its gauge, which indicates its thickness/diameter. The length of the screw is indicated by the second number, which tells you how many threads are pressed into a one-inch length of the screw. You can also determine a screw’s threads per inch (TPI) by counting the number of thread peaks along a one-inch length of the screw. The major diameter of a screw is determined by multiplying its gauge and threads per inch.
Imperial-system screw packaging typically lists the screw gauge and length, but doesn’t include the threads per inch. Screws that do list the threads per inch usually list it in the form of a decimal, with the first number representing the gauge and the second number being the threads per inch. For example, a #10 screw has a major diameter of 1/2 inch and a threads per inch of 35. Screws that don’t include the threads per inch will often list only the gauge and length, like 10 x 2 inches.