When you’re working on a project that requires fasteners, the right size screw will be crucial for success. The wrong screw will cause the joint to flex and weaken over time, while the correct one will hold it together with strength. The type of project you’re working on will also play a role in the head and shaft size required. Getting the size right can be difficult, especially with different systems of measurement used for screw sizes. To understand the basics, you’ll need a ruler or tape measure and a screw gauge. A caliper can offer the highest level of precision, providing measurements down to the nearest thousandths of an inch or millimeter.
A screw’s diameter and thread pitch are two of the most important dimensions to know when selecting a fastener. Diameter refers to the major diameter of a screw, while thread pitch is the distance between adjacent screw threads. There are several different systems for determining screw size, including the imperial system and the metric system. The table below provides the equivalant measurements in inches (fractions and decimals) and millimeters for each screw size.
The first number listed in the chart refers to the screw’s major diameter. If you’re using a countersunk screw, make sure to measure from the top of the head. For non-countersunk screws, measure from where the flat side underneath the head rests on the material’s surface. The second number indicates the screw’s thread count, and the third is the shaft length in inches. #6 screw diameter