Screws come in many sizes for an endless array of tasks, from fastening wood to installing metal roofs. Selecting the wrong size screw can cause damage to your project or affect its structural integrity. Knowing three essential measurements—gauge, length and threads per inch (TPI)—will help you avoid such pitfalls.
The gauge of a screw is its thickness, which determines how far it can be driven into the material. A screw with a thicker gauge can penetrate deeper into material, while a screw with a thinner gauge will need to be driven less deeply to provide a strong hold.
Screw threads are generated by subtractive techniques, such as cutting [single-point, taps and dies, milling, rolling], molding and casting; and newer additive methods such as powder metallurgy. These processes can create a wide variety of thread profiles, from coarse to fine. The screw thread standardization committees of the United States, Canada and Great Britain standardized an inch-based system of inch-sized screws in the early 20th century, called the Unified Thread Series.
In the UTS system, a screw's gauge and threads per inch are designated by its major diameter and thread pitch, respectively. For example, a screw labeled 6-32 x 1 1/2" means the screw has a #6 gauge with 32 threads per inch—twice the number of threads on a normal wood screw.
In the metric system, screw sizes are specified by the major diameter in millimeters and the thread pitch in threads per centimeter. A metric screw's pitch is measured by counting the distance between two adjacent thread peaks, which requires a precision measuring tool such as a caliper. 3/8 inch to mm
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