Screws are great for holding materials together, but using the wrong type can cause them to bend or break. The best way to avoid this is to measure the thickness of your material and use a screw that is appropriate for it. To find the right screw size, you will need to know a few different measurements, such as the screw gauge, threads per inch and length. In this article, we’ll show you how to measure each of these to help ensure your screws have the proper strength and are able to grip securely.

Screw Gauge
The first number in a screw’s measurement is called the gauge, and it refers to the outer diameter of the threads. This is the same for both inch and metric systems of measurement, but is usually prec

eded by the letter M in the case of metric screws. Engineering Toolbox has a helpful chart that lists the screw gauges and their decimal equivalents.

The next number is the threads per inch, which is the number of threads in one inch of a screw or bolt. This is often followed by a number that designates the head-bore or shank-hole diameter, which is the smooth part of the screw above the threads. Screws with a countersunk head (also known as oval countersunk screws) require a smaller diameter than standard round or hex screw heads.1/4 to mm