How to use the tap drill size calculator
The hole you drill before tapping sets both the strength of the thread and how hard the tap has to work. Too large and the thread strips easily, too small and tapping torque climbs until the tap snaps off in the part. Pick a UNC or UNF size and a target percent of thread and this tool returns the theoretical drill diameter along with the standard chart drill.
The formula is tap drill = major diameter minus 1.299 divided by threads per inch, times the percent of thread divided by 100. A 1/4-20 at 75 percent works out to 0.2013 in, which is exactly why the chart calls for a #7 drill at 0.201 in. The same relation gives 5/16 in for 3/8-16 and 27/64 in for 1/2-13.
Pushing engagement from 75 to 100 percent buys only about 5 percent more joint strength while more than doubling tapping torque, so 75 percent is the shop default for steel. Stainless, titanium and deep blind holes are often dropped to 60 or 65 percent to protect the tap, while soft aluminum can carry a higher figure.
Note that these values apply to cut taps. Form or roll taps displace material instead of cutting it and need a noticeably larger hole, typically around 65 percent of thread, so check the tap manufacturer chart before switching tap types. Confirm the final size on a test piece whenever thread strength is critical.
Frequently asked questions
Tap drill diameter equals the major diameter minus 1.299 divided by threads per inch, multiplied by the target percent of thread over 100. For a 1/4-20 at 75 percent that gives 0.2013 in, which is the standard #7 drill at 0.201 in.
Going from 75 to 100 percent thread adds only about 5 percent to joint strength while more than doubling tapping torque and the risk of a broken tap. Steel is usually tapped at 75 percent and stainless or deep blind holes at 60 to 65 percent.