What size is a 10.3 mm drill bit?

A 10.3 mm drill bit is 0.4055 in in diameter, which is 10.300 mm.

What thread does a 10.3 mm drill bit tap?

A 10.3 mm drill bit is the tap drill for M12×1.75 at 75% requested thread engagement, where it leaves 74.8% thread with a cut tap.

What fraction is a 10.3 mm drill bit closest to?

A 10.3 mm drill bit (0.4055 in) sits closest to the 13/32" fractional drill at 0.4063 in, a difference of 0.0007 in.

Metric drill · 10.3 mm

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

https://shopmath.org/drills/metric/10-3mm · data verified August 1, 2026

A 10.3 mm drill bit is 0.4055 in (10.300 mm) in diameter. It is the tap drill for M12×1.75 at 75% thread engagement with a cut tap, where it leaves 74.8% thread. The nearest fractional size is 13/32" at 0.4063 in. Diameters are nominal.

Series: Metric
Designation: 10.3 mm
Inch: 0.4055
mm: 10.300
Standard: ANSI/ASME B94.11M

Metric Drill · 10.3 mm · nominal diameter

0.4055 in

Millimetre: 10.300mm
Taps: M12×1.75

Nominal diameter. As a tap drill it leaves 74.8% thread in M12×1.75 with a cut tap.

10.3 mm Drill Bit Size in Inches and Millimetres

The nominal diameter of a 10.3 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.

10.3 mm drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.4055 in
Diameter, millimetre	10.300 mm
Cross-sectional area	0.12915 in²

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

Threads a 10.3 mm Drill Bit Taps

Every thread in this reference whose tap drill is the 10.3 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 10.3 mm drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
M12×1.75	ISO Metric Coarse	75%default	74.8
M12×1.5	ISO Metric Fine	85%	87.2

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

10.3 mm Drill Bit Equivalents in Every Other Drill Series

The closest size to the 10.3 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 10.3 mm drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.1775
Y	Letter	0.4040	10.262	−0.0015
13/32"	Fractional	0.4063	10.319	+0.0007

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 10.3 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 10.3 mm drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
10.2 mm	Metric	0.4016	10.200
Y	Letter	0.4040	10.262
10.3 mm	Metric	0.4055	10.300
13/32"	Fractional	0.4063	10.319
10.4 mm	Metric	0.4094	10.400

Sources and Method for the 10.3 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. M12×1.75 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 10.3 mm Drill Bit Reference

Citation

ShopMath. "10.3 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/metric/10-3mm

BibTeX

@misc{shopmath-drills-metric-10-3mm,
  title        = {10.3 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/metric/10-3mm}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/metric/10-3mm

Common Questions About the 10.3 mm Drill Bit

What size is a 10.3 mm drill bit?: A 10.3 mm drill bit is 0.4055 in in diameter, which is 10.300 mm.
What thread does a 10.3 mm drill bit tap?: A 10.3 mm drill bit is the tap drill for M12×1.75 at 75% requested thread engagement, where it leaves 74.8% thread with a cut tap.
What fraction is a 10.3 mm drill bit closest to?: A 10.3 mm drill bit (0.4055 in) sits closest to the 13/32" fractional drill at 0.4063 in, a difference of 0.0007 in.

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