When selecting screws for woodworking projects, understanding the differences between screw sizes is essential to ensuring that they can hold and anchor the project’s materials. Using the wrong types of screws can cause them to bend or break, and can also lead to unsightly gaps in the final product. The proper screw size depends on the thickness of the materials and the weight of the project, and can be determined by the screw’s gauge, threading or shaft length.

Screws are usually listed in two dimensions, their gauge and shaft length (excluding the head). Most commonly, screw packaging will list the diameter first, followed by the length, with the thread count coming last. For instance, a package with the measurements 6-40 x 2” indicates that the screw has a #6 diameter, and is 18 inches long. In the imperial system, the thread count is generally given in fractions of an inch, while in the metric system the gauge is given in millimeters.

Screws are available in a wide variety of head configurations, such as slotted, Phillips and square heads. Additionally, they can come in different types of threading and material construction. Different types of woods require slightly different threading, while engineered materials like OSB may be better suited for specialized screws with coarser threads to minimize stripping. Wood screws can also be categorized by their shank-hole diameter, which is the width of the smooth portion of the screw that goes into the shank hole.   5/16 inch to mm