When working with screws, determining the correct size can mean the difference between a successful project and one that ends up needing to be redone. Choosing the right screw also involves knowing what type of material you’re working with and how thick it is, as different screw sizes are intended to go into different types of materials. Identifying the proper screw can be complicated, as measurements are often based on fractions of an inch and decimal places. Using precise tooling such as a vernier caliper is recommended, but can be difficult without this tool.
Generally speaking, when a screw is labeled with metric system measurements the first number (before the hyphen) refers to the diameter of the thread and the second number refers to the length of the shaft. So, a screw labeled as M2 x 40mm has a thread diameter of 4.2 mm and a shaft length of 40 mm. For a screw head that will protrude from the surface, such as a round or oval countersunk head, the shaft length is measured from the base of the head to the underside.
In the United States, screw sizes are based on both gauge and length. The gauge number, which is typically represented by a # or “G” followed by a fraction of an inch, corresponds to a certain major diameter and pitch of the screw. A simple formula to calculate the major diameter of a screw size greater than or equal to 0 is to multiply the screw gauge number by 0.013 and add 0.060. #6 screw diameter