Tap Drill Calculator — UNC, UNF, NPT & Metric
Search the published shop chart and select a thread to get its 75% tap drill and decimal equivalent instantly.
Formula and worked example
Frequently asked questions
What is the tap drill size for 3/8-16 UNC?
Use a 5/16-inch drill (0.3125 inch) for approximately 75% thread engagement.
What is the tap drill size for 1/4-20 UNC?
Use a #7 drill, with a 0.2010-inch decimal diameter.
How do I choose a metric tap drill?
For common metric coarse threads, subtracting pitch from major diameter closely matches a 75% chart size. For example, M6 × 1.0 uses a 5.0 mm drill.
Can I use the straight-thread formula for NPT?
No. NPT is tapered and uses dedicated pipe-tap drill sizes, so select the chart entry rather than applying the UNC or metric formula.
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