Knowing how to read screw size chart is essential when picking out the right fasteners for a project. It can seem confusing at first, but once you understand what each number stands for, reading screw callouts becomes easy.
Screws are often measured in imperial or metric systems and sometimes both at the same time. The letter “M” on a screw package indicates that the measurements are in metric units, and if the numbers begin with “ISO,” they are in imperial system units. For example, a “10 x 2” screw means that the screw has a diameter of 10 mm and is 2 inches long.
Most screw sizes are listed with two or three numbers. The first number is the screw gauge and specifies the diameter of the threads. The next number is the screw length and the last is the threads per inch, or TPI. Screws are usually divided into coarse and fine thread series, so the TPI indicates which series the screw belongs to. The screw length is measured from the bottom of the head to the tip of the shaft, and if the screw has a rounded head or countersunk head, the measurement is taken where the oval top meets the angled countersink and where it would rest on the surface.
Threads per inch is the number of thread peaks that fit in one-inch length. This is measured with a thread gage, which has strips of metal with different-sized threads cut into them. You systematically work your way through the strips to find the size of the thread that matches the thread on the screw. The number of threads per inch is then multiplied by the screw major diameter to find the number of threads in a one-inch length.
A screw’s pitch is the distance between adjacent thread peaks at the same point. Screws with a small thread pitch are coarse and those with a large thread pitch are fine. Sometimes the pitch isn’t listed because coarse screws are more common than fine screws.
Metric screw sizes are measured in millimeters and use a similar formula to determine screw thread size. You also need to know the major diameter and count the threads per centimeter (which is more difficult to measure accurately since you need a caliper). For example, a screw with a major diameter of 4 mm has a thread pitch of 1 mm.
When choosing a metric screw, always check the major diameter and threads per centimeter first. If the diameter is too small, it will break easily. Then, you can look at the screw length and TPI to make sure it’s a suitable length for your application. It’s also important to pay attention to whether the screw is coarse or fine, which refers to its strength and resistance to damage.