Tap & Drill Size Chart

The tap drill for 1/4-20 UNC is a #7 bit (0.2010″ / ~5.1 mm) at 75% thread engagement. For 1/4-28 UNF, use #3 (0.2130″). Clearance for a 1/4″ bolt: close fit F (0.257″), free fit H (0.266″) — see the chart columns.

M10×1.5 (coarse, default when the print says M10) → 8.5 mm (0.3346″). M10×1.25 (fine) → 8.8 mm. M10×1.0 (extra fine) → 9.0 mm. Metric fine pitches use a larger tap drill than coarse because the threads are shallower.

M6×1.0 coarse → 5.0 mm (~0.1969″) at 75%. M6×0.75 fine → slightly larger hole (~5.2 mm). Search M6 in the chart to see every pitch option.

UNC (Unified National Coarse) has fewer threads per inch — easier hand assembly and more forgiving of damaged threads. UNF (Unified National Fine) has more TPI — stronger in thin materials and more resistant to loosening from vibration. UNC is the default for general fastening; UNF is common in automotive and aerospace.

Coarse pitch (e.g. M10×1.5) is the default on drawings. Threads are deeper and farther apart. Fine pitch (M10×1.25 or M10×1.0) has shallower, closer threads — stronger in thin walls and better against vibration, but harder to tap. Use coarse unless the drawing specifies fine.

Close fit drills a hole just barely larger than the bolt — minimal wiggle, best for precise alignment (dowel pins, shoulder screws). Free fit drills a larger hole so the bolt drops in easily — best for covers, brackets, and quick assembly. These apply to through holes without threads, not tap drills.

Use 75% thread for most applications — about 90% of full-thread strength with much lower tap breakage risk. For aluminum or brass, 60–65% still holds well and extends tap life. Going above 75% adds little strength but dramatically increases tapping force.

Sometimes, if the imperial size is close — e.g. 21/64″ (0.3281″) is near 8.5 mm for M10×1.5. "Close" means slightly different engagement and higher tapping force. For production, use the correct metric drill from the chart.

Too small → higher torque, more heat, broken taps (especially in stainless or deep blind holes). Too large → shallow threads that strip under load. The chart values balance thread strength and tapping ease at 75% engagement.

Yes — filter NPT for tapered pipe thread tap drills (1/8 NPT, 1/4 NPT, etc.) per ANSI B1.20.1. Clearance columns show — for NPT because pipe threads seal by taper, not bolt clearance holes. Always verify with the tap manufacturer for pipe applications.

The alternate option is the nearest stocked standard drill rounded up from the calculated diameter — the bit you actually grab from the tool crib. The decimal diameter is the exact theoretical size before rounding to a catalog drill.