Tap Drill & Clearance Hole CalculatorTAP DRILL & CLEARANCE HOLE

1. 1Choose the thread system: ISO metric or inch UNC / UNF.
2. 2Pick the thread from the dropdown — fine metric pitches such as M10×1.25 are listed too.
3. 3Adjust the thread engagement if needed; around 65 % for hard materials or deep blind holes.
4. 4Read the tap drill, clearance holes (fine / medium / coarse) and counterbore, and copy them with 📋 into your drawing or job sheet.
5. 5For 3D-printed parts use the self-tap or heat-set insert hole, or click "Open in 3D generator" to make the matching screw and nut.

An internal thread's strength depends on how deep the tap cuts, and that is set by the drilled hole. A smaller hole raises the engagement but the tapping torque rises much faster. For metric threads the rule "tap drill = major diameter − pitch" gives about 77 % engagement; for inch threads the drill is found from Ø = D − 0.01299 × % / TPI and rounded to the nearest standard drill. The basic minor diameter D1 = d − 1.082532P is shown too, so you can check the hole against the 6H minor-diameter tolerance.

Parts the screw passes through need a clearance hole, not a thread — otherwise both threads fight each other and the joint never clamps. ISO 273 defines fine, medium and coarse clearance; medium is the usual design choice. To sink a socket head flush, the counterbore diameter follows DIN 974-1 and its depth is slightly more than the head height. Inch clearances use D + 1/64, 1/32 and 1/16 in as an approximation of ASME B18.2.8 close / normal / loose — check the standard for production drawings.

Fine threads rarely print well on FDM. The common options are: a hole of about 0.9·d so the screw self-taps (simple, but wears with repeated assembly); a heat-set insert pressed in with a soldering iron (durable and reusable); or a hex pocket for a captive nut. Hole sizes still depend on the printer, filament and layer height, so print a test coupon first. Parts made by Orinovate in SLS / MJF or CNC can be tapped directly.

Metric tap drill (ISO 2306): Ø = d − P  (≈ 77 % thread engagement)
Standard drill: d − P rounded to the next standard drill size (0.1 mm steps ≥ M3, 0.05 mm below; DIN 336), e.g. M8 → 6.8, M12 → 10.2
Drill for a given engagement: Ø = d − 0.01299 · % · P   (P = 1 / TPI for inch)
Engagement of a drill: % = (d − Ø) / (0.01299 · P)
Basic minor diameter (internal): D1 = d − 1.082532 · P
Clearance holes ISO 273: fine / medium / coarse; inch ≈ D + 1/64, 1/32, 1/16 in (close / normal / loose, ≈ ASME B18.2.8)
Socket-head counterbore (DIN 974-1): Ø per table, depth ≈ k + 0.5 mm (k = d for ISO 4762)
3D print: self-tapping hole ≈ 0.9 · d (0.85–0.95 · d); heat-set insert hole = vendor value (reference)

Results are for design reference only; confirm against supplier catalogues and the applicable standards before manufacturing.