What size is a X drill bit?

A X drill bit is 0.3970 in in diameter, which is 10.084 mm.

What is the metric equivalent of a X drill bit?

The closest metric drill to a X drill bit (10.084 mm) is 10.1 mm, which is 10.100 mm — 0.0006 in larger.

What fraction is a X drill bit closest to?

A X drill bit (0.3970 in) sits closest to the 25/64" fractional drill at 0.3906 in, a difference of 0.0064 in.

Letter drill · X

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

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

A X drill bit is 0.3970 in (10.084 mm) in diameter. The nearest fractional size is 25/64" at 0.3906 in. Diameters are nominal.

Series: Letter
Designation: X
Inch: 0.3970
mm: 10.084
Standard: ANSI/ASME B94.11M

Letter Drill · X · nominal diameter

0.3970 in

Millimetre: 10.084mm

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

X Drill Bit Size in Inches and Millimetres

The nominal diameter of a X 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.

X drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.3970 in
Diameter, millimetre	10.084 mm
Cross-sectional area	0.12379 in²

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

Threads a X Drill Bit Taps

Every thread in this reference whose tap drill is the X 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 X drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
7/16"-28 UNEF	Unified Extra Fine (UNEF)	80%	87.3
7/16"-28 UNEF	Unified Extra Fine (UNEF)	85%	87.3
7/16"-20 UNF	Unified Fine (UNF)	60%	62.4
7/16"-20 UNF	Unified Fine (UNF)	65%	62.4

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

X Drill Bit Equivalents in Every Other Drill Series

The closest size to the X 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 X drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.1690
25/64"	Fractional	0.3906	9.922	−0.0064
10.1 mm	Metric	0.3976	10.100	+0.0006

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 X 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 X drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
25/64"	Fractional	0.3906	9.922
10 mm	Metric	0.3937	10.000
X	Letter	0.3970	10.084
10.1 mm	Metric	0.3976	10.100
10.2 mm	Metric	0.4016	10.200

Sources and Method for the X 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.
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 X Drill Bit Reference

Citation

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

BibTeX

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

Permanent URL

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

Common Questions About the X Drill Bit

What size is a X drill bit?: A X drill bit is 0.3970 in in diameter, which is 10.084 mm.
What is the metric equivalent of a X drill bit?: The closest metric drill to a X drill bit (10.084 mm) is 10.1 mm, which is 10.100 mm — 0.0006 in larger.
What fraction is a X drill bit closest to?: A X drill bit (0.3970 in) sits closest to the 25/64" fractional drill at 0.3906 in, a difference of 0.0064 in.

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