Tapped Hole Guide: Sizing, Callouts and Hole Sizes
A tapped hole is a hole that has been drilled first and then threaded with a tap. "Tapped" and "threaded" are often used interchangeably, but on drawings they mean different things — and the drill size you start from is what decides whether the finished thread is strong or strips.
How a tapped hole is sized
inch threads: tap drill = major Ø − (pitch × 0.75) metric threads: tap drill = major Ø − pitch NPT pipe threads: sized to the pipe taper, not % engagement
The drill leaves the right amount of material for the tap to cut ~75% of a full thread — the standard strength point. Smaller hole: the tap stalls or snaps. Larger hole: shallow threads strip under load. For the full worked examples, every thread size below links to its own reference page with the formula applied.
Common hole sizes for tapping
The most-searched "what size hole for X tap" lookups, answered here and explained in depth on each thread's page:
What is a tapped hole?
A tapped hole is created in two steps: a hole is drilled at a specific diameter, then a tap — a fluted, hardened cutting tool — is run into it to cut internal threads. The metal the tap removes leaves the thread flanks that a screw engages with.
Tapped vs threaded hole: the terms overlap in everyday speech. In strict usage, threaded hole is the umbrella term — a tapped hole is one specific way of making a threaded hole (cutting with a tap rather than forming with a roll tap, machining on a mill, or moulding). A tapped hole is always threaded; a threaded hole is not always tapped.
Blind hole vs through hole
- Through hole: the hole exits the far side. Chips fall away, the tap can run out the bottom, and full-depth threads are straightforward. Easiest case.
- Blind hole: the hole stops inside the part. Chips have nowhere to go, so they pack and can break the tap — use spiral-flute taps that pull chips back out, drill deeper than the needed thread depth (the first 1–2 threads near the chamfer are usually incomplete), and back off to clear chips.
- Bottoming tapped hole: a blind hole whose threads run almost to the bottom, finished with a bottoming tap after a standard taper/plug tap has done the upper threads.
"Bottoming tapped hole vs tapped hole" — the distinction is depth utilisation: a standard tapped hole leaves unusable thread at the start of the hole and above the bottom; a bottoming callout means the design needs usable threads right to the floor.
Tapped hole callout
On engineering drawings a tapped hole is specified so the shop needs no guessing. A typical inch callout:
1/4-20 UNC × 0.50 DEEP, TAPPED HOLE DRILL Ø 0.201 (letter #7) × 0.62 DEEP 4 PLACES
The callout names the thread, the usable thread depth, the drill diameter and the drill depth — drill depth is always deeper than thread depth so the tap's chamfered lead threads have somewhere to go. Metric callouts read the same way: M6×1.0 × 12 DEEP with the drill size and depth below it. CAD tools (SolidWorks hole wizard etc.) generate this callout automatically; the numbers behind it are the same ones on our size pages.