Taps, drills, and threads

Tap drills — pick a thread family and size, set the target thread engagement, and tick the drill series you own. The tool finds the drill that lands closest to the target and lists its neighbours with the engagement each would give.

Cutting taps: % thread = 76.98 × (D − drill) / P, so drill = D − %·P / 76.98. 75% is the classic target; 60–65% taps much more easily in stainless and titanium for very little strength loss.
Forming (roll) taps displace metal: hole = D − 0.0068 × % × P (P in inches), typically 55–65%.
Pipe threads (BSPP/G, BSPT/R, NPT) use shop-standard drill sizes; ream taper threads for best results.

Clearance & counterbore — close/normal/loose clearance holes (ISO 273 for metric; the ASME B18.2.8 pattern for inch), socket head cap screw counterbores, and flat-head countersink diameters (90° metric, 82° inch).

Drill chart — every number (#1–#80), letter (A–Z), fractional (1/64″ steps to 1½″) and metric (0.1 mm to 13 mm, 0.5 mm to 38 mm) drill with decimal equivalents; search it, or type a size to find the nearest drills.

Pitch diameter & 3-wire — for 60° threads, E = D − 0.649519·P, best wire = 0.57735·P, and the measurement over wires M = E − 0.866025·P + 3W.

Data sources: drill decimal equivalents from the standard ANSI drill chart; thread dimensions from ISO 261/262 and ASME B1.1; clearance holes from ISO 273 and ASME B18.2.8; socket screw heads from ISO 4762/10642 and ASME B18.3 (typical shop values).

Starting points — verify with your tooling manufacturer and the fastener standard you are working to. Tap makers often publish their own recommended drill for each tap.

Share link copies a URL that opens this tool exactly as you have it set up. It carries the thread, tap and drill-chart settings. Opening a link replaces your own saved settings for the tool; a Restore my settings button brings them back. To write a link by hand, use fam, units — for example /tools/drill-tap/?fam=UNC&units=in.

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