What size is a 21/64" drill bit?

A 21/64" drill bit is 0.3281 in in diameter, which is 8.334 mm. It is the 21/64 in fractional size.

What is the metric equivalent of a 21/64" drill bit?

The closest metric drill to a 21/64" drill bit (8.334 mm) is 8.3 mm, which is 8.300 mm — 0.0014 in smaller.

Fractional drill · 21/64

21/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/fractional/21-64 · data verified August 1, 2026

A 21/64" drill bit is 0.3281 in (8.334 mm) in diameter. Diameters are nominal.

Series: Fractional
Designation: 21/64
Inch: 0.3281
mm: 8.334
Standard: ANSI/ASME B94.11M

Fractional Drill · 21/64 · nominal diameter

0.3281 in

Millimetre: 8.334mm
Fraction: 21/64in

Nominal diameter. A worn or run-out drill cuts oversize, which leaves less thread in a tapped hole than the table says.

21/64" Drill Bit Size in Inches and Millimetres

The nominal diameter of a 21/64" 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.

21/64" drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.3281 in
Diameter, millimetre	8.334 mm
Fraction	21/64 in
Cross-sectional area	0.08456 in²

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

Threads a 21/64" Drill Bit Taps

Every thread in this reference whose tap drill is the 21/64" 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 21/64" drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
3/8"-24 UNF	Unified Fine (UNF)	85%	86.6
3/8"-16 UNC	Unified Coarse (UNC)	60%	57.7

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

21/64" Drill Bit Equivalents in Every Other Drill Series

The closest size to the 21/64" 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 21/64" drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.1001
Q	Letter	0.3320	8.433	+0.0039
8.3 mm	Metric	0.3268	8.300	−0.0014

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 21/64" 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 21/64" drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
P	Letter	0.3230	8.204
8.3 mm	Metric	0.3268	8.300
21/64"	Fractional	0.3281	8.334
8.4 mm	Metric	0.3307	8.400
Q	Letter	0.3320	8.433

Sources and Method for the 21/64" Drill Bit Size

Drill diametercomputed

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

Computed from the standard's formulas, then checked against published tables.

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.

Formula

Fractional drill diameter

Ø  =  21/64 in  =  0.3281 in

Fractional drills are exact fractions of an inch in 1/64 in steps; the decimal is the fraction evaluated, not a measurement.

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.
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 21/64" Drill Bit Reference

Citation

ShopMath. "21/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/fractional/21-64

BibTeX

@misc{shopmath-drills-fractional-21-64,
  title        = {21/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/fractional/21-64}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/fractional/21-64

Common Questions About the 21/64" Drill Bit

What size is a 21/64" drill bit?: A 21/64" drill bit is 0.3281 in in diameter, which is 8.334 mm. It is the 21/64 in fractional size.
What is the metric equivalent of a 21/64" drill bit?: The closest metric drill to a 21/64" drill bit (8.334 mm) is 8.3 mm, which is 8.300 mm — 0.0014 in smaller.

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