When choosing and purchasing screws, knowing what the size is can help you get the job done correctly. Screw sizes are typically described as three figures – the gauge, the threads per inch (TPI) and the length either in inches or millimeters (mm). The gauge is determined by the basic outer diameter of the screw i.e. the major diameter and the threads are added to this, resulting in the total thread size of the screw.
Most commonly used in the U.S, a screw’s size is designated by the gauge and the number of threads per inch. Screws with coarser thread counts are called Unified Coarse Thread (UNC) and those with a finer thread count are known as Unified Fine Thread (UNF).
For wood screws, the diameter of the screw at the point just below the head is often referred to as the root diameter. Depending on the application, the thickness of the material and the amount of weight that will be attached to the screw, a different screw size may be required. For example, thicker materials and heavy attachments require a larger gauge while fine pieces will require a smaller gauge.
When choosing the right screw for a project, you must also select a screw head style. Square and torque heads are great fits for power drills while flatheads and Phillips styles work well in many applications. The type of head you choose will also impact the shear and tensile strength that a screw can support. 3.5 mm to inches