What size is a 21.5 mm drill bit?

A 21.5 mm drill bit is 0.8465 in in diameter, which is 21.500 mm.

What fraction is a 21.5 mm drill bit closest to?

A 21.5 mm drill bit (0.8465 in) sits closest to the 27/32" fractional drill at 0.8438 in, a difference of 0.0027 in.

Metric drill · 21.5 mm

21.5 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/metric/21-5mm · data verified August 1, 2026

A 21.5 mm drill bit is 0.8465 in (21.500 mm) in diameter. The nearest fractional size is 27/32" at 0.8438 in. Diameters are nominal.

Series: Metric
Designation: 21.5 mm
Inch: 0.8465
mm: 21.500
Standard: ANSI/ASME B94.11M

Metric Drill · 21.5 mm · nominal diameter

0.8465 in

Millimetre: 21.500mm

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

21.5 mm Drill Bit Size in Inches and Millimetres

The nominal diameter of a 21.5 mm 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.

21.5 mm drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.8465 in
Diameter, millimetre	21.500 mm
Cross-sectional area	0.56273 in²

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

Threads a 21.5 mm Drill Bit Taps

Every thread in this reference whose tap drill is the 21.5 mm 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 21.5 mm drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
M24×3	ISO Metric Coarse	65%	64.1

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

21.5 mm Drill Bit Equivalents in Every Other Drill Series

The closest size to the 21.5 mm 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 21.5 mm drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.6185
Z	Letter	0.4130	10.490	−0.4335
27/32"	Fractional	0.8438	21.431	−0.0027

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 21.5 mm 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 21.5 mm drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
21.25 mm	Metric	0.8366	21.250
27/32"	Fractional	0.8438	21.431
21.5 mm	Metric	0.8465	21.500
21.75 mm	Metric	0.8563	21.750
55/64"	Fractional	0.8594	21.828

Sources and Method for the 21.5 mm 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.

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 21.5 mm Drill Bit Reference

Citation

ShopMath. "21.5 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/metric/21-5mm

BibTeX

@misc{shopmath-drills-metric-21-5mm,
  title        = {21.5 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/metric/21-5mm}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/metric/21-5mm

Common Questions About the 21.5 mm Drill Bit

What size is a 21.5 mm drill bit?: A 21.5 mm drill bit is 0.8465 in in diameter, which is 21.500 mm.
What fraction is a 21.5 mm drill bit closest to?: A 21.5 mm drill bit (0.8465 in) sits closest to the 27/32" fractional drill at 0.8438 in, a difference of 0.0027 in.

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