Screws hold materials together with strength, unlike nails, which may come loose over time. But choosing the correct length, gauge and head type for a project can be difficult. The following guidelines can help.
Screw sizes are measured according to the Unified Thread Standard (UTS). The first number of a screw’s UTS designation refers to its major diameter, or outside thread diameter; a screw with a smaller major diameter is often called a “fine” thread series. A screw with a larger major diameter is usually called a “coarse” thread series.
The second number of a screw’s UTS rating is the number of threads per inch, or TPI. To determine a screw’s thread pitch, simply count the number of thread peaks that are visible over an inch length of the screw. For example, a screw with a major diameter of 1/4″ and a thread pitch of 20 threads per inch is labeled a #4 wood screw.
In addition to these three fundamental qualities, the size of a screw is determined by its length, or shaft, in inches. A screw less than half the length of the material it’s being installed into will not anchor properly; one that is much longer than this can be dangerous.
Screws are also available with different heads to suit their specific applications. For instance, drywall screws have a curve on the junction between the head and shaft to prevent tearing in drywall. Screws designed for use with hardwoods have a coarser thread than those made for softerwoods to minimize stripping. 1/4 in to mm