Screw Sizes and Threads
Screws are used for a wide variety of construction projects, from woodworking to metal roofing. But choosing the right screw size can make all the difference in your project. Choose a screw that’s too long and it will pull out of the material; one that’s too short won’t anchor it properly. The screw’s diameter and thread pattern should also match the material on which you are working. For example, drywall screws have a curved junction that prevents tearing while wood screws have coarse and fine threads for different uses.
Screw sizes are generally labeled with two numbers: the gauge and the threads per inch. The first number, which is called the major diameter, represents the outside diameter of the screw. For example, a 1/4″ screw will have a gauge of 12. The second number, which is the number of threads per inch, tells you how many thread peaks are on an inch of screw length. For instance, a 2-1/4″ screw has 32 threads per inch and is a #8 screw.
For more information about screw dimensions, check out this article from Engineering Toolbox. They have a handy chart that shows you the decimal equivalent of each screw gauge.
Screws can be measured in either the imperial or metric system. The UTS (Unified Thread Standard) screw measurement system is the most common in North America, but a growing number of projects are using ISO metric screw threads. This is especially true on the international level where the metric system has begun to supplant inch-based measurements. 3/8 inch to mm