7 months ago

Screws come in many types and sizes for a multitude of construction tasks. However, selecting the wrong screw can split wood or compromise the soundness of a structure. Measuring the diameter and length of a screw is easy enough, but knowing how to determine the thread size can be tricky. This article is designed to help you understand the three fundamental measurements that every tradesperson should know.

There are two different systems for determining screw sizes: the imperial system that uses inches and the metric system that uses millimeters. For fasteners that use the inch series, the diameter of a screw is usually stated first followed by the number of threads per inch or TPI. This can be determined by counting the number of peaks on a one-inch length of the screw and dividing the result by the major diameter.

Metric screws are typically listed with a major diameter in mm followed by the thread pitch also in mm. The thread pitch is measured by calculating the distance between two adjacent thread peaks on a one-inch length and then multiplying the results by the number of threads per inch. This can be a difficult measurement to make accurately and requires the use of a caliper.

The last important measurement of a screw is the shaft length, which is usually stated in either inches or decimal format. Depending on the application, you may need to select a longer or shorter screw than the measurement that you are currently using. A general rule of thumb is to select a screw that is at least half as long as the material it will be used in.
1/4 in to mm