Screws come in many different sizes to suit a variety of construction tasks. Choosing the wrong size can split wood or even affect the soundness of a structure. As such, it’s important to understand how to measure the diameter and length of a screw. This knowledge allows you to choose the right screws for your projects, saving time and money on wasted supplies.
The most basic method of determining a screw size is to use a thread pitch gauge. This involves lining up the screw with a ruler or tape measure to see how many threads fit in an inch. In the case of a standard wood screw, that would mean counting how many threads are visible between the crest and top of one side and the bottom cavity of the other side. This is also referred to as the major diameter of the screw.
Another measurement is the length of a screw from its head to its tip. This is important when selecting a screw for use with a drill. It will help you determine how far to drill so that the screw won’t protrude beyond the surface and damage it. The length of a screw is often listed on the packaging as a combination of the head and shaft length in inches, for example, 6-32 x 1-1/4″.
A third measurement is the diameter of a screw’s threads, called the minor diameter. This is usually measured by using a thread gauge, which measures the diameter of a theoretical cylinder that passes through the center of the threads on both sides. Alternatively, you can simply line up the screw with a ruler or tape to measure how wide the minor diameter is. pilot hole for 3/8 lag screw