What size is a 13.1 mm drill bit?

A 13.1 mm drill bit is 0.5157 in in diameter, which is 13.100 mm.

What fraction is a 13.1 mm drill bit closest to?

A 13.1 mm drill bit (0.5157 in) sits closest to the 33/64" fractional drill at 0.5156 in, a difference of 0.0001 in.

Metric drill · 13.1 mm

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

https://shopmath.org/drills/metric/13-1mm · data verified August 1, 2026

A 13.1 mm drill bit is 0.5157 in (13.100 mm) in diameter. The nearest fractional size is 33/64" at 0.5156 in. Diameters are nominal.

Series: Metric
Designation: 13.1 mm
Inch: 0.5157
mm: 13.100
Standard: ANSI/ASME B94.11M

Metric Drill · 13.1 mm · nominal diameter

0.5157 in

Millimetre: 13.100mm

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

13.1 mm Drill Bit Size in Inches and Millimetres

The nominal diameter of a 13.1 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.

13.1 mm drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.5157 in
Diameter, millimetre	13.100 mm
Cross-sectional area	0.20891 in²

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

13.1 mm Drill Bit Equivalents in Every Other Drill Series

The closest size to the 13.1 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 13.1 mm drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.2877
Z	Letter	0.4130	10.490	−0.1027
33/64"	Fractional	0.5156	13.097	−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 13.1 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 13.1 mm drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
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
13.3 mm	Metric	0.5236	13.300

Sources and Method for the 13.1 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.

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 13.1 mm Drill Bit Reference

Citation

ShopMath. "13.1 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/metric/13-1mm

BibTeX

@misc{shopmath-drills-metric-13-1mm,
  title        = {13.1 mm Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/metric/13-1mm}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/metric/13-1mm

Common Questions About the 13.1 mm Drill Bit

What size is a 13.1 mm drill bit?: A 13.1 mm drill bit is 0.5157 in in diameter, which is 13.100 mm.
What fraction is a 13.1 mm drill bit closest to?: A 13.1 mm drill bit (0.5157 in) sits closest to the 33/64" fractional drill at 0.5156 in, a difference of 0.0001 in.

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