The following formulas can aid in finding the right drill size for tapping. The formulas work well as a starting point. Other factors may need to be taken into consideration in choosing the right drill for the job, such as the type of drill or tap being used, or the hardness of the material being cut. The formula generates a theoretical drill size and looks for the closest matching standard drill. Tap drill charts often show only one tap drill size for producing an approximate 75 percent thread. Tap tool wear can be reduced utilizing a lower percent of thread. The formulas below have options for varying percentages.
1 Select Size Category.2 Select Tap Size.3 Select % of full thread.