Tap Drill Size
Tap drill for any UNC, UNF, or metric thread at the thread engagement you choose.
The theoretical drill diameter, the nearest standard drill (number, letter, fraction, or mm) with the engagement it actually gives, and the alternatives either side.
Example: An M10 × 1.5 thread at 75 % engagement takes an 8.5 mm tap drill; 1/4-20 UNC takes a #7 (0.201 in) drill.
Computed from the thread,
not copied from a chart.
So you can choose the engagement, not just 75 %.
How the drill size is calculated
For a cut tap, the drill sets the minor diameter and therefore the percentage of full thread. Inch: drill = major − 0.01299 × (% thread) ÷ TPI. Metric: drill = major − 0.01299 × (% thread) × pitch. The constant comes from the basic thread depth 0.6495 P (ASME B1.1 / ISO 68). The theoretical diameter is then matched to the nearest standard drill — number, letter, 1/64 in fractions, or 0.1 mm steps — and the engagement that drill actually gives is shown, plus the alternatives either side. At 75 % this reproduces the classic charts (1/4-20 → #7, 3/8-16 → 5/16, M8 → 6.8 mm).
Why charts sometimes differ
Many metric charts use the shortcut drill = major − pitch (about 77 %), so they list 10.2 mm for M12 × 1.75 while 75 % computes to 10.3 mm; both appear here with their real engagement. Between 60 % and 75 % the thread loses little strength but the tap cuts much easier — a common choice in tough materials. Roll-form (fluteless) taps need a larger drill and are not covered.
What this utility does not do
No pipe threads (NPT/BSP), no roll-form taps, no clearance holes, no tolerance classes. The thread and drill tables are the textbook standards, pending a reviewer's check against ASME B1.1 and ISO 261. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.
SOURCES
- ASME B1.1 and ISO 261 thread series; drill sizes cross-checked against two independent public tables
Last reviewed 17 September 2026. How results are checked: How we verify.