How to Measure a Screw's Head Diameter and Shank Diameter

 If you're not an expert on fasteners, figuring out what type of screw you need for your next project can be confusing. While you can always take a guess, knowing how to measure a screw's head diameter and shank diameter will help you avoid a costly mistake. This article also explains the difference between imperial and metric screw sizes, so you can choose the right one for your project.

Screws are measured according to different systems, including the Unified Thread Standard (UTS), which uses inch measurements and separates screw thread series into coarse and fine. Measuring a screw's major diameter is easy, but determining the threads per inch (TPI) can be tricky because of the minute spacing between each of the peaks. To get a precise measurement, you'll need to use a caliper or screw gauge.

Most of us are familiar with imperial screw sizes, as they are still used in the United States and many other countries. However, as the world moves toward using metric measuring standards, fewer and fewer supplies will be sold in Imperial units. For this reason, it's important to understand the metric system so you can easily purchase the correct screws for your projects.

There are several types of screw heads, including phillips, square, and hex. Most screw heads are designed to fit a specific screw driver or bit. This allows the user to insert the screw into wood without having to remove the driver or bit first. Other screw characteristics include length and whether or not the screw is countersunk. This last feature is used when a screw will sit below the surface of the wood and requires a deeper cut than a standard screw would.

A screw's length depends on the grain orientation of the board into which it will be inserted. Screws that will be used to join boards across the grain do not need as long of a screw length as those being installed into end grain. It is important to have enough length so the screw passes completely through the thicker piece of lumber, but not so much that it is pulling against the board on the other side of the grain.

For most wood applications, the head of a screw is less important than the shank or body. The head diameter is roughly double the screw shank diameter, though this isn't always the case. The shank may be threaded, partially-threaded, or not threaded at all.

In the UTS screw system, screw sizes are given by their major diameter and the number of threads per inch (TPI). The easiest way to determine your screw size is to count the number of peaks in an inch-long section of the screw's threads. You can then multiply the threads per inch number by the major diameter to get your screw size. This method can be inaccurate if you don't use a caliper or screw gauge, but it is still the most common method for determining screw size.#5 screw diameter