Whether you’re sorting out a drawer of mismatched screws or choosing the right screw for an upcoming project, understanding screw sizes is key. Screws are sized by gauge (diameter), length and threads per inch (TPI). They come in many shapes, forms and styles to suit a variety of different applications such as woodworking, drywall and metal roofing. Using the wrong type or size can damage a project, split a piece of wood or impact the strength and soundness of a building.
The most common method to measure the diameter of a screw is by its gauge size, which is often labeled in inches and fractions of an inch. For example, a screw that has a major diameter of 1/4″ is labeled as #6 and increases by 1/16″ with each larger size.
Screws also have a thread pattern that determines their ability to fasten to materials like wood or drywall. Tighter threads allow screws to grab more material, while looser threads give more flexibility for removal or adjustment if needed.
For woodworkers, this can be confusing as plans, instructions and other woodworking sources often reference specific screw types and sizes. Fortunately, the folks at Engineering Toolbox have put together a helpful chart that shows you what each of the gauge numbers and TPI values mean when it comes to screw size. The chart includes both US and metric decimal equivalents, and it also lists the decimal equivalents of a screw size to its closest fractional equivalent. 5/16 to mm