A tap drill size is associated with a tap and drill set, which is used for metal threading; different size drills are recommended based on media type and thickness, according to Irwin Tools. Tap drill size charts are available online, offered by resources like the North Carolina State University (NCSU) Physics Department and Newman Tools Inc.
The tap drill size charts from North Carolina State University feature standard, imperial and metric tap drills as well as a sizing chart of pipe drills. The information provided also alludes to the tolerance of the drill and hole, which can help assure quality tap drill practices, as stated by NCSU. Additionally, Newman Tools Inc. provides size recommendations for 75 percent threads, and even a calculation tool that users can use to compute their own measurements.