The drill size chart is a table list standard size drill bits in several measurement systems, including fractional, metric, wire gauge number, and letter. Fractional sizes are measured in inches, while metric sizes are measured in millimeters. The wire gauge and letter systems refer to tool diameters that increase as the wire gauge decreases from #107 to #1 and then continues from A to Z. The decimal equivalents of the diameters are shown in both English and Metric units. When manufacturing custom CNC machining parts, holes with screw threads are often required for fastening the finished part or facilitating the building. One of the most common ways to produce the thread holes is to drill a hole of the right size with a drill bit and tap it with a tap. People would need a drill size chart and tap size chart to determine the proper tap drill for a certain thread size or proper tap for an existing hole.