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Fractional drill · 31/64

31/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/fractional/31-64 · data verified August 1, 2026

A 31/64" drill bit is 0.4844 in (12.303 mm) in diameter. It is the tap drill for 9/16"-12 UNC at 75% thread engagement with a cut tap, where it leaves 72.2% thread. Diameters are nominal.

Series
Fractional
Designation
31/64
Inch
0.4844
mm
12.303
Standard
ANSI/ASME B94.11M

Fractional Drill · 31/64 · nominal diameter

0.4844 in

Millimetre
12.303mm
Fraction
31/64in
Taps
9/16"-12 UNC

Nominal diameter. As a tap drill it leaves 72.2% thread in 9/16"-12 UNC with a cut tap.

31/64" Drill Bit Size in Inches and Millimetres

The nominal diameter of a 31/64" 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.

31/64" drill bit — decimal inch and millimetre diameter
PropertyValue
Diameter, decimal inch0.4844 in
Diameter, millimetre12.303 mm
Fraction31/64 in
Cross-sectional area0.18427 in²

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

Threads a 31/64" Drill Bit Taps

Every thread in this reference whose tap drill is the 31/64" 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 31/64" drill bit, by thread engagement
ThreadSeriesThread % asked forThread % this drill leaves
9/16"-12 UNCUnified Coarse (UNC)75%default72.2
1/2"-28 UNEFUnified Extra Fine (UNEF)50%33.7
9/16"-12 UNCUnified Coarse (UNC)65%72.2
9/16"-12 UNCUnified Coarse (UNC)70%72.2

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

31/64" Drill Bit Equivalents in Every Other Drill Series

The closest size to the 31/64" 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 31/64" drill bit
PropertyPropertyDiameterProperty
DrillSeriesinmmDifference, in
#1Number0.22805.791−0.2564
ZLetter0.413010.490−0.0714
12.3 mmMetric0.484312.300−0.0001

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 31/64" 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 31/64" drill bit, by diameter
PropertyPropertyDiameter
DrillSeriesinmm
12.2 mmMetric0.480312.200
12.3 mmMetric0.484312.300
31/64"Fractional0.484412.303
12.4 mmMetric0.488212.400
12.5 mmMetric0.492112.500

Sources and Method for the 31/64" 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  =  31/64 in  =  0.4844 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.
  • Tap drill usage assumes cut taps. 9/16"-12 UNC 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 31/64" Drill Bit Reference

Citation

ShopMath. "31/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/fractional/31-64

BibTeX

@misc{shopmath-drills-fractional-31-64,
  title        = {31/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/fractional/31-64}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/fractional/31-64

Common Questions About the 31/64" Drill Bit

What size is a 31/64" drill bit?

A 31/64" drill bit is 0.4844 in in diameter, which is 12.303 mm. It is the 31/64 in fractional size.

What thread does a 31/64" drill bit tap?

A 31/64" drill bit is the tap drill for 9/16"-12 UNC at 75% requested thread engagement, where it leaves 72.2% thread with a cut tap.

What is the metric equivalent of a 31/64" drill bit?

The closest metric drill to a 31/64" drill bit (12.303 mm) is 12.3 mm, which is 12.300 mm — 0.0001 in smaller.

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