BenchReference

Q Drill Bit

Decimal inches 0.3320″
Millimeters 8.433 mm
System Letter

Nearest equivalents in other systems

System Nearest drill Size Difference
Metric 8.4 mm 0.3307″ / 8.4 mm −0.033 mm (−0.0013″)
Fractional inch 21/64″ 0.3281″ / 8.334 mm −0.098 mm (−0.0039″)
Number (wire gauge) #1 0.2280″ / 5.791 mm −2.642 mm (−0.1040″)
−0.675 mm+0.675 mmQO−0.406 mmP−0.229 mmR+0.178 mmS+0.406 mm21/64″−0.098 mm8.4 mm−0.033 mm← #1 −2.642 mmtick position = true diameter · letter neighbors above, equivalents below · window ±0.67 mm
Positive difference = cuts a slightly larger hole than Q. Sizes beyond the window are listed with arrows — they are not close substitutes.

Tap drill for

Thread Major diameter Pitch
3/8″-24 UNF 0.3750″ / 9.525 mm 24 TPI

The standard chart recommendation (≈75% thread engagement). Application-specific factors — material, hole depth, tap type — can justify a different drill.

Neighboring letter sizes

Drill Decimal in mm vs Q
O 0.3160 8.026 −0.406 mm
P 0.3230 8.204 −0.229 mm
Q 0.3320 8.433 —
R 0.3390 8.611 +0.178 mm
S 0.3480 8.839 +0.406 mm
Data provenance. ASME B94.11M, Twist Drills (standard drill size series) (ASME). Machinery’s Handbook — tap drill charts (Industrial Press). Millimeter and inch values are converted with the exact definition 1 in = 25.4 mm; the canonical dimension for this letter size is 0.3320″. See all data sources.