Screw Size and Length – How to Measure a Screw

When working with screws, the right size can make or break your project. A wrong-sized screw can split the material it’s screwing into or affect the overall soundness of the construction. Choosing the correct screw for your materials and tasks is essential, and knowing how to measure them correctly can help you save money on extras that aren’t needed. Screws are measured in three different ways: gauge, length and threads per inch (TPI). Understanding these measurements will ensure you get the best results when using them.

To determine the screw’s gauge, you must first measure its diameter. This is typically listed on the screw’s packaging as a number, or in the imperial system, it may be noted as “#” followed by a number. In the metric system, this measurement is shown as a decimal value that corresponds to the diameter of the screw’s threaded portion.

Once you know the screw’s diameter, you can calculate its length. This is a bit more complicated and depends on the type of work you’re doing with it. For example, if you’re screwing into wood that will be subject to lateral or withdrawal pressure, you’ll need to choose a longer screw to support the forces involved.

The screw’s head size should also be taken into account when calculating its length. For screw heads that protrude from the surface, use a stick or straw to mark where it hits the far side of the material you’re working with. Place a bolt scale on top of the mark and move it until you find a matching number.  #6 screw size