Knowing the right screw size for a specific project can help you ensure that your screws will fit into whatever material you’re using them on, hold up the weight of the load they’ll be holding and remain in place without bending or breaking. Screws come in a huge variety of lengths, diameters and threads and are designed for different applications. Choosing the best screws for the job requires careful consideration of the thickness, density and strength of the substrate you’re working with as well as the type of screw head, drive and thread.
The easiest way to measure a screw’s diameter is by measuring from the crest, or peak, of its threads. This measurement is known as the major diameter, and it can be used to determine a screw’s gauge number. The gauge number corresponds to a fraction of an inch, and you can use a screw gauge guide to match up the “#” with the appropriate diameter measurements.
Another important measurement to consider when determining screw size is the number and spacing of its threads, called the thread pitch. You can find this by lining up a screw against a ruler or tape measure and counting the space between its threads. Screws with larger thread pitches have coarser threads, while those with smaller thread pitches have finer ones. Screws sold with imperial system measurements usually list the gauge first and the length second, while those with metric measurements will typically also list their thread count, which may be listed as either a number or a percentage of the thread’s diameter. #6 screw size