The diameter of a screw is one of its most critical dimensions, and knowing how to determine it is essential to ensuring the correct fit in the material you’re drilling into. A screw that is too small won’t hold, while a screw that is too large can cause the threads to split or break. The right size will ensure that your screw fits snugly but can still be easily tightened or loosened.
The screw diameter is the measurement of the widest point on a screw’s shaft, and it can be expressed in either millimeters (mm) or inches (in). The best way to measure a screw’s diameter is with a caliper, as this tool provides high-precision measurements. Be sure to measure the shaft at its widest point, avoiding the threads, to get the most accurate reading.
Screws also are measured in terms of their pitch and thread size, but these measurements are less intuitive and can be confusing to new users. Thread size refers to the distance between adjacent threads, and it can be measured with a ruler or with a pitch gauge, which is a device that has stepped wires that you can place against the screw to measure the pitch.
To make it easier for you to understand these measurements, we’ve created a chart that shows the corresponding measurements in both imperial and metric systems. We’ve also included the ANSI and FED-STD-H28 specifications for additional details about screw diameter and other dimensional information. #6 screw diameter