🔧 Tap Drill Size

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · A documented rule of thumb, pinned by automated tests · how PhDino checks its numbers

Choosing the right drill diameter to bore a hole before cutting internal threads with a tap.

Cutting an internal thread starts with a hole slightly smaller than the thread's major (outer) diameter, sized so the tap has enough material to cut a properly formed thread — but not so little material that the tap binds, breaks, or produces threads engaged well beyond what's actually needed.

Thread engagement is described as a percentage of full thread depth. Counterintuitively, more isn't simply better: 100% thread engagement requires a very tight drill size that dramatically increases tapping torque and the risk of breaking a tap, for very little practical strength gain over a more modest 65–75% engagement — a range most general-purpose fastening applications target.

A widely used shop rule of thumb for standard 60° unified threads is that drilling a hole one full thread pitch smaller than the major diameter gives approximately 75% thread engagement — simple enough to do at the bench without a reference table, and accurate enough for most general work.

Key formula

Pitch = 1 ÷ threads per inch
Tap drill diameter ≈ major diameter − pitch × (percent thread ÷ 75)

Notes & limitations

• This gives a calculated diameter — actual shop practice is to match it to the nearest standard drill bit size (commonly graduated in 64ths of an inch, or a numbered/lettered drill index for finer steps), not to source an exact custom reamer.
• Blind holes (that don't go all the way through) need extra drill depth beyond the thread length itself, since the tap's conical lead-in chamfer needs somewhere to go and doesn't cut full-form threads for its own length.