Screws are used in countless construction tasks, and using the wrong size can cause serious problems. For example, screws that are too big for a hole can crack the material in which they’re placed, while screws that are too small won’t provide enough hold and may fail under stress or over time. Measuring a screw’s diameter, length, and thread pitch is important to help ensure that you’re getting the right fit for each application.
In most cases, the gauge of a screw is listed first on the packaging with the length and threads per inch (TPI) following. This is how you can quickly and easily identify what size screw you’re buying. The gauge is the basic size of the screw’s outer threads, and typically corresponds to a number from #0 through to #12 in imperial system measurements. In Austrlia, screw sizes are typically listed by the metric system with an M prefix indicating the diameter and threads per inch, while the length is stated in either inches or millimeters depending on the standard used.
The diameter of a screw measures the width of its shaft at its widest point. This measurement is particularly critical when working with wood screws, as a screw that is too wide for a hole can split the board into which it’s being inserted. When measuring a screw’s diameter, it’s best to use a caliper to achieve high-precision results. For wood screws, the major diameter is defined as a measure between the crest of one side of the thread and the root of the other. 5/16 lag bolt pilot hole