What size is a 1/8" drill bit?

A 1/8" drill bit is 0.1250 in in diameter, which is 3.175 mm. It is the 1/8 in fractional size.

What is the metric equivalent of a 1/8" drill bit?

The closest metric drill to a 1/8" drill bit (3.175 mm) is 3.2 mm, which is 3.200 mm — 0.0010 in larger.

Fractional drill · 1/8

1/8" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/fractional/1-8 · data verified August 1, 2026

A 1/8" drill bit is 0.1250 in (3.175 mm) in diameter. Diameters are nominal.

Series: Fractional
Designation: 1/8
Inch: 0.1250
mm: 3.175
Standard: ANSI/ASME B94.11M

Fractional Drill · 1/8 · nominal diameter

0.1250 in

Millimetre: 3.175mm
Fraction: 1/8in

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

1/8" Drill Bit Size in Inches and Millimetres

The nominal diameter of a 1/8" 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.

1/8" drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.1250 in
Diameter, millimetre	3.175 mm
Fraction	1/8 in
Cross-sectional area	0.01227 in²

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

1/8" Drill Bit Equivalents in Every Other Drill Series

The closest size to the 1/8" 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 1/8" drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#30	Number	0.1285	3.264	+0.0035
A	Letter	0.2340	5.944	+0.1090
3.2 mm	Metric	0.1260	3.200	+0.0010

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 1/8" 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 1/8" drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
#31	Number	0.1200	3.048
3.1 mm	Metric	0.1220	3.100
1/8"	Fractional	0.1250	3.175
3.2 mm	Metric	0.1260	3.200
#30	Number	0.1285	3.264

Sources and Method for the 1/8" 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.

Formula

Fractional drill diameter

Ø  =  1/8 in  =  0.1250 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.
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 1/8" Drill Bit Reference

Citation

ShopMath. "1/8" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/drills/fractional/1-8

BibTeX

@misc{shopmath-drills-fractional-1-8,
  title        = {1/8" Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/drills/fractional/1-8}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/drills/fractional/1-8

Common Questions About the 1/8" Drill Bit

What size is a 1/8" drill bit?: A 1/8" drill bit is 0.1250 in in diameter, which is 3.175 mm. It is the 1/8 in fractional size.
What is the metric equivalent of a 1/8" drill bit?: The closest metric drill to a 1/8" drill bit (3.175 mm) is 3.2 mm, which is 3.200 mm — 0.0010 in larger.

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