If you’re planning to work with screws in your next project, it’s important to choose the right ones. Choosing the wrong size can cause damage or lead to failure of your work. Fortunately, there are a few guidelines you can follow to ensure you’re getting the right screw for the job.
The first number on a screw or bolt’s callout is the diameter of the threaded part (not including the head). It’s usually expressed in millimetres, such as 5.0 mm or 4 mm. The second number is the number of threads per inch. The higher the number, the finer the thread. The lower the number, the coarser the thread. The difference between coarse and fine threads is what distinguishes one screw series from another.
Some screws also have a third number which is the pitch of the threads. This is determined by counting the number of thread peaks on an inch length of screw. This is different from threads per inch which measures the spacing of threads and how dense they are.
Once you’ve figured out the diameter and pitch of a screw, you can calculate the major diameter using the following formula: Major diameter = Screw number x 0.013 in + 0.060 in. If a screw’s callout contains the letters M or ST, this indicates that it is a metric screw. These screws have a special type of tip that allows them to be screwed into material without needing a pilot hole drilled beforehand. screw size chart
