What size is a 33/64" drill bit?

A 33/64" drill bit is 0.5156 in in diameter, which is 13.097 mm. It is the 33/64 in fractional size.

What thread does a 33/64" drill bit tap?

A 33/64" drill bit is the tap drill for 9/16"-24 UNEF at 75% requested thread engagement, where it leaves 86.6% thread with a cut tap. It is the default tap drill for 2 threads in this reference.

What is the metric equivalent of a 33/64" drill bit?

The closest metric drill to a 33/64" drill bit (13.097 mm) is 13.1 mm, which is 13.100 mm — 0.0001 in larger.

Fractional drill · 33/64

33/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/fractional/33-64 · data verified August 1, 2026

A 33/64" drill bit is 0.5156 in (13.097 mm) in diameter. It is the tap drill for 9/16"-24 UNEF at 75% thread engagement with a cut tap, where it leaves 86.6% thread. Diameters are nominal.

Series: Fractional
Designation: 33/64
Inch: 0.5156
mm: 13.097
Standard: ANSI/ASME B94.11M

Fractional Drill · 33/64 · nominal diameter

0.5156 in

Millimetre: 13.097mm
Fraction: 33/64in
Taps: 9/16"-24 UNEF

Nominal diameter. As a tap drill it leaves 86.6% thread in 9/16"-24 UNEF with a cut tap.

33/64" Drill Bit Size in Inches and Millimetres

The nominal diameter of a 33/64" 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.

33/64" drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.5156 in
Diameter, millimetre	13.097 mm
Fraction	33/64 in
Cross-sectional area	0.20881 in²

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

Threads a 33/64" Drill Bit Taps

Every thread in this reference whose tap drill is the 33/64" 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 33/64" drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
9/16"-24 UNEF	Unified Extra Fine (UNEF)	75%default	86.6
9/16"-18 UNF	Unified Fine (UNF)	75%default	65.0
9/16"-24 UNEF	Unified Extra Fine (UNEF)	80%	86.6
9/16"-24 UNEF	Unified Extra Fine (UNEF)	85%	86.6
9/16"-18 UNF	Unified Fine (UNF)	60%	65.0
9/16"-18 UNF	Unified Fine (UNF)	65%	65.0
9/16"-18 UNF	Unified Fine (UNF)	70%	65.0
9/16"-12 UNC	Unified Coarse (UNC)	50%	43.3

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

33/64" Drill Bit Equivalents in Every Other Drill Series

The closest size to the 33/64" 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 33/64" drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.2876
Z	Letter	0.4130	10.490	−0.1026
13.1 mm	Metric	0.5157	13.100	+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 33/64" 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 33/64" drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
12.9 mm	Metric	0.5079	12.900
13 mm	Metric	0.5118	13.000
33/64"	Fractional	0.5156	13.097
13.1 mm	Metric	0.5157	13.100
13.2 mm	Metric	0.5197	13.200

Sources and Method for the 33/64" 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  =  33/64 in  =  0.5156 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"-24 UNEF 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 33/64" Drill Bit Reference

Citation

ShopMath. "33/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/fractional/33-64

BibTeX

@misc{shopmath-drills-fractional-33-64,
  title        = {33/64" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/fractional/33-64}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/fractional/33-64

Common Questions About the 33/64" Drill Bit

What size is a 33/64" drill bit?: A 33/64" drill bit is 0.5156 in in diameter, which is 13.097 mm. It is the 33/64 in fractional size.
What thread does a 33/64" drill bit tap?: A 33/64" drill bit is the tap drill for 9/16"-24 UNEF at 75% requested thread engagement, where it leaves 86.6% thread with a cut tap. It is the default tap drill for 2 threads in this reference.
What is the metric equivalent of a 33/64" drill bit?: The closest metric drill to a 33/64" drill bit (13.097 mm) is 13.1 mm, which is 13.100 mm — 0.0001 in larger.

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