Knowing the dimensions of your screw is critical to correctly identifying the correct size and ensuring compatibility with the material you’re fastening. These dimensions are usually represented by the ‘gauge number’ in the Imperial system, or in fractions of an inch (in – mm for metric). For best results, we recommend using a calibrated tool such as a digital or vernier caliper to ensure accurate measurements.
Diameter is the width of a screw shaft at its widest point. This measurement determines how deep a screw will fit into the material you’re fastening. It corresponds to the first number in a screw size chart. Screws are available in imperial sizes 2 through 16, and metric sizes from M2 to M16.
Thread size, also called pitch, is the distance between the thread peaks on a screw. It is measured by counting the number of thread peaks along a one-inch length. For example, a screw with 20 threads per inch has a coarse thread, while a screw with 24 threads per inch has finer, tighter threads.
Length is determined by measuring from the end of the shaft to the underside of a screw head for all heads except oval head screws. This is the standard method for determining screw size and is most often used.
For oval head screws, the diameter and length measurement are made from the center of the screw’s round, flat top to the tip of the head. This measurement is referred to as the ‘nominal’ diameter and is sometimes used to simplify the screw size designation to omit unwieldy decimals. screw diameters