Tap Drill Size
Tap drill for any UNC, UNF, or metric thread at the thread engagement you choose.
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 four anonymous usage counts as the rest of the site apply.