Knowing the right tap drill size to use for your screws or bolts is nearly impossible to determine without the use of a good tap drill chart. A tap drill chart is an essential reference tool that every machinist should have readily accessible and at hand.
How to Read a Tap Drill Chart
A good tap drill chart like the printable PDF version below typically has three main columns or sections. The first column lists the standard drill sizes as a number (a fraction or letter drill size, for instance). The second column will list the inch decimal equivalent of the drill diameter of the the tap drill you need to use. The third and final column lists the tap size and the threads per inch that will be required.
Standard sizes are split into two classes – small diameter sets and large diameter sets. Sizes less than 1/4" utilize a numbering system rather than fractional measurements, and range from #0-#12, with #0 being the smallest and #12 being the largest standard size before you reach 1/4". For sizes of 1/4" or more, fractional sizes based on the screw diameter are used. Also listed on a good tap drill chart, you will find reference to 'threads per inch' (T.P.I), which refers to the thread count of the screw. The term 'major diameter' refers to the distance between the two outer ridges of the threading, while the term 'minor diameter' refers to the distance between the two inner ridges of the threading.
Download and print the tap drill chart pdf for a quick reference table that will help you select the right tap and drill for your application, every time! Our comprehensive chart includes standard sizes, metric sizes, plug tap sizes and an 'at a glance' listing of the most popular tap drill sizes.