What size is a Q drill bit?

A Q drill bit is 0.3320 in in diameter, which is 8.433 mm.

What thread does a Q drill bit tap?

A Q drill bit is the tap drill for 3/8"-24 UNF at 75% requested thread engagement, where it leaves 79.4% thread with a cut tap.

What is the metric equivalent of a Q drill bit?

The closest metric drill to a Q drill bit (8.433 mm) is 8.4 mm, which is 8.400 mm — 0.0013 in smaller.

What fraction is a Q drill bit closest to?

A Q drill bit (0.3320 in) sits closest to the 21/64" fractional drill at 0.3281 in, a difference of 0.0039 in.

Letter drill · Q

Q Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/letter/q · data verified August 1, 2026

A Q drill bit is 0.3320 in (8.433 mm) in diameter. It is the tap drill for 3/8"-24 UNF at 75% thread engagement with a cut tap, where it leaves 79.4% thread. The nearest fractional size is 21/64" at 0.3281 in. Diameters are nominal.

Series: Letter
Designation: Q
Inch: 0.3320
mm: 8.433
Standard: ANSI/ASME B94.11M

Letter Drill · Q · nominal diameter

0.3320 in

Millimetre: 8.433mm
Taps: 3/8"-24 UNF

Nominal diameter. As a tap drill it leaves 79.4% thread in 3/8"-24 UNF with a cut tap.

Q Drill Bit Size in Inches and Millimetres

The nominal diameter of a Q drill bit. Drill sizes are nominal — a worn drill, run-out or a soft material that closes in behind the cut all move the finished hole.

Q drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.3320 in
Diameter, millimetre	8.433 mm
Cross-sectional area	0.08657 in²

1 in = 25.4 mm exactly, so the two columns are the same value in different units, not two measurements.

Threads a Q Drill Bit Taps

Every thread in this reference whose tap drill is the Q drill bit, and the thread engagement it leaves. The engagement asked for and the engagement the drill actually produces are different numbers — that gap is why both columns are here.

Threads tapped with a Q drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
3/8"-24 UNF	Unified Fine (UNF)	75%default	79.4
3/8"-24 UNF	Unified Fine (UNF)	80%	79.4
3/8"-16 UNC	Unified Coarse (UNC)	50%	53.0

Values are for cut taps. Rows marked default are the shop-default engagement for that thread.

Q Drill Bit Equivalents in Every Other Drill Series

The closest size to the Q drill bit in each of the other three indexes, with the difference in inches. None of them is the same drill — check the difference before substituting one into a tapped hole.

Nearest number, letter, fractional and metric equivalents to a Q drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.1040
21/64"	Fractional	0.3281	8.334	−0.0039
8.4 mm	Metric	0.3307	8.400	−0.0013

A positive difference means the equivalent drill is larger, which leaves less thread in a tapped hole. Sizes without a link are stored for the nearest-drill calculation but do not carry their own page.

Drill Sizes Either Side of the Q Drill Bit

Ordered by diameter across all four indexes, so the next size up may be a letter, a fraction or a metric drill rather than the next one in this series.

Standard drill sizes adjacent to a Q drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
21/64"	Fractional	0.3281	8.334
8.4 mm	Metric	0.3307	8.400
Q	Letter	0.3320	8.433
8.5 mm	Metric	0.3346	8.500
8.6 mm	Metric	0.3386	8.600

Sources and Method for the Q Drill Bit Size

Drill diametercompiled

Number/letter: bd_warehouse drill_sizes.csv (Apache-2.0) cross-checked against Wikipedia Drill bit sizes (CC BY-SA 4.0). Fractional: computed 1/64 in steps per ANSI B94.11M. Metric: computed BS 328 increment pattern.

Verified against Wikipedia Drill bit sizes (CC BY-SA 4.0); 31/31 checks passed

Compiled from independent public sources that agree cell-for-cell.

Tap drill usagecomputed+compiled

bd_warehouse drill_sizes.csv (Apache-2.0) + Wikipedia Drill bit sizes (CC BY-SA 4.0); fractional and metric computed from ANSI B94.11M / BS 328 increment patterns

Verified against drills: 31 checks passed across 1 harness(es)

Part computed from the standard's formulas, part compiled from agreeing public sources.

Assumptions

Diameters are nominal. Drill manufacturing tolerance, wear and run-out all cut oversize, and every one of those moves a tapped hole toward less thread than the table says.
Tap drill usage assumes cut taps. 3/8"-24 UNF at 75% requested engagement is what this drill is normally bought for.
Dataset scope: Number sizes #61-#80 are stored but tagged demandTier 'extended' — query-space-map.md Appendix A found the tail thinning in the 60s. Sizes below #80 and metric sizes above 25.0 mm excluded.

Data last verified

August 1, 2026

How this data is built and checked →

How to Cite This Q Drill Bit Reference

Citation

ShopMath. "Q Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/letter/q

BibTeX

@misc{shopmath-drills-letter-q,
  title        = {Q Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/letter/q}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/letter/q

Common Questions About the Q Drill Bit

What size is a Q drill bit?: A Q drill bit is 0.3320 in in diameter, which is 8.433 mm.
What thread does a Q drill bit tap?: A Q drill bit is the tap drill for 3/8"-24 UNF at 75% requested thread engagement, where it leaves 79.4% thread with a cut tap.
What is the metric equivalent of a Q drill bit?: The closest metric drill to a Q drill bit (8.433 mm) is 8.4 mm, which is 8.400 mm — 0.0013 in smaller.
What fraction is a Q drill bit closest to?: A Q drill bit (0.3320 in) sits closest to the 21/64" fractional drill at 0.3281 in, a difference of 0.0039 in.

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