The best size wood screw for a given project depends on the type of joint you’re making. For example, if you’re installing cabinet knobs or pulling together two pieces of solid-core plywood to make a bookcase, you’ll probably want to use a #8 wood screw, which has a diameter of about 5/32 inch. If you’re attaching furniture to a wall or framing a shed, on the other hand, you might need a heavier-duty wood screw with a larger diameter. Choosing the right size wood screw also depends on the type of material you’re working with.
There are several factors to consider when selecting a screw, including the diameter, threads, and head type. The most common wood screws are self-tapping, meaning they have a sharp tip that taps into the bottom of a hole (or screw cavity) to form an internal thread and prevent the screw from splitting when you drive it in. Other types of wood screws require you to pre-drill a pilot hole before inserting the screw.
Screw sizing is determined by three numbers: the gauge, or outside diameter; the number of threads per inch, or TPI; and the length, or thread pitch. The number of threads in a one-inch section of a screw is specified right after the gauge, typically separated by a hyphen. For instance, a screw with a gauge of “8” has 32 threads in an inch and will need a drill bit with a TPI of about 8 turns for every inch of length.
The number of turns is what determines the depth a screw will go into a board, with more threads per inch creating deeper holes and smaller ones producing shallower holes. The height of the thread is also important, since this relates to how much force will be needed to turn the screw. A screw with a higher number of threads will need more torque to penetrate the surface, while a lower number will be easier to turn and requires less force.
The final factor to consider is the screw’s head, which is usually rounded or flat, and may have a decorative design or be a convenient handle for driving the screw into place. Some screw heads are also designed to hold nuts or washers, which can add a decorative element to your project and help keep the fastener in place. You might also prefer a screw with a removable head to quickly and easily remove it for cleaning or repair work. This is particularly useful when working with upholstered furniture, which can be damaged by a pointed screw head. Most types of screws have a screw length indicated in inches, but some are available in metric measurements as well. A handy toolbox or tool shed will have a conversion table that lists the metric equivalents of standard screw sizes. A convenient online calculator is also available. For more information, visit Engineering Toolbox’s screw size chart, which shows all major screw dimensions and their decimal counterparts. #8 screw diameter