Screw size is one of the most important factors in choosing the right fasteners for a job. It determines the load bearing capacity of a screw and whether it can properly penetrate the material you’re working with. Using the wrong fastener can result in stripped threads or protruding tips that may cause damage or pose safety risks.
Screw sizes are sized according to a combination of two things: their major diameter and thread count. The major diameter is the largest diameter of a screw’s thread, measured from the crest (top) of one side to the crest of the other. Screws with larger major diameters are also referred to as “gauges.” The smaller the gauge, the bigger the screw; for example, a #12 screw has a much greater major diameter than a #8 screw. When screws are sized with numbers less than 1/8″, they’re usually given a decimal equivalent; Engineering Toolbox has a handy chart that lists screw gauges and their decimal counterparts.
When describing screw sizes, manufacturers will usually include the gauge first and then list their length second. For imperial system screws, they’ll usually include a thread count in between, such as 10 x 2″ (a #10 screw has a major diameter of 1/2″ and a thread count of 35-40 threads per inch). Metric systems use a different measuring standard that includes both the thread count and thread pitch; for example, a metric screw with a major diameter of 6 mm will have a pitch of 1 mm. #6 screw size