What size is a 21 mm drill bit?

A 21 mm drill bit is 0.8268 in in diameter, which is 21.000 mm.

What thread does a 21 mm drill bit tap?

A 21 mm drill bit is the tap drill for M24×3 at 75% requested thread engagement, where it leaves 77.0% thread with a cut tap.

What fraction is a 21 mm drill bit closest to?

A 21 mm drill bit (0.8268 in) sits closest to the 53/64" fractional drill at 0.8281 in, a difference of 0.0014 in.

Metric drill · 21 mm

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

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

A 21 mm drill bit is 0.8268 in (21.000 mm) in diameter. It is the tap drill for M24×3 at 75% thread engagement with a cut tap, where it leaves 77.0% thread. The nearest fractional size is 53/64" at 0.8281 in. Diameters are nominal.

Series: Metric
Designation: 21 mm
Inch: 0.8268
mm: 21.000
Standard: ANSI/ASME B94.11M

Metric Drill · 21 mm · nominal diameter

0.8268 in

Millimetre: 21.000mm
Taps: M24×3

Nominal diameter. As a tap drill it leaves 77.0% thread in M24×3 with a cut tap.

21 mm Drill Bit Size in Inches and Millimetres

The nominal diameter of a 21 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 mm drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.8268 in
Diameter, millimetre	21.000 mm
Cross-sectional area	0.53686 in²

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

Threads a 21 mm Drill Bit Taps

Every thread in this reference whose tap drill is the 21 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 mm drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
M24×3	ISO Metric Coarse	75%default	77.0
M22×1.5	ISO Metric Fine	50%	51.3
M24×3	ISO Metric Coarse	80%	77.0

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

21 mm Drill Bit Equivalents in Every Other Drill Series

The closest size to the 21 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 mm drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.5988
Z	Letter	0.4130	10.490	−0.4138
53/64"	Fractional	0.8281	21.034	+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 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 mm drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
13/16"	Fractional	0.8125	20.637
20.75 mm	Metric	0.8169	20.750
21 mm	Metric	0.8268	21.000
53/64"	Fractional	0.8281	21.034
21.25 mm	Metric	0.8366	21.250

Sources and Method for the 21 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.
Tap drill usage assumes cut taps. M24×3 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 21 mm Drill Bit Reference

Citation

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

BibTeX

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

Permanent URL

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

Common Questions About the 21 mm Drill Bit

What size is a 21 mm drill bit?: A 21 mm drill bit is 0.8268 in in diameter, which is 21.000 mm.
What thread does a 21 mm drill bit tap?: A 21 mm drill bit is the tap drill for M24×3 at 75% requested thread engagement, where it leaves 77.0% thread with a cut tap.
What fraction is a 21 mm drill bit closest to?: A 21 mm drill bit (0.8268 in) sits closest to the 53/64" fractional drill at 0.8281 in, a difference of 0.0014 in.

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