Screw sizes are generally written in the following format:
Diameter x Length
The diameter is the measurement of the screw's widest part, while the length is the distance from the top of the screw head to the tip of the screw. The diameter is usually measured in millimeters (mm) or inches, while the length is typically measured in millimeters or inches.
For example, a screw size might be written as "4 x 40 mm," which means that the screw has a diameter of 4 millimeters and a length of 40 millimeters.
There are also some screws that have a letter preceding the size, such as "M4 x 40 mm." This letter indicates the thread pitch or the distance between threads. For example, an M4 screw has a thread pitch of 0.7 millimeters.
To measure the size of a screw, you can use a screw gauge or a caliper. A screw gauge is a tool that has a series of holes with corresponding screw sizes inscribed around the edges. To use a screw gauge, you simply place the screw in the hole that best fits its diameter. A caliper is a precision measuring tool that can be used to measure the diameter and length of a screw with greater accuracy.