What size is a O drill bit?

A O drill bit is 0.3160 in in diameter, which is 8.026 mm.

What is the metric equivalent of a O drill bit?

The closest metric drill to a O drill bit (8.026 mm) is 8 mm, which is 8.000 mm — 0.0010 in smaller.

What fraction is a O drill bit closest to?

A O drill bit (0.3160 in) sits closest to the 5/16" fractional drill at 0.3125 in, a difference of 0.0035 in.

Letter drill · O

O Drill Bit Size — Decimal Inch, Millimetre and Tap Sizes

https://shopmath.org/drills/letter/o · data verified August 1, 2026

A O drill bit is 0.3160 in (8.026 mm) in diameter. The nearest fractional size is 5/16" at 0.3125 in. Diameters are nominal.

Series: Letter
Designation: O
Inch: 0.3160
mm: 8.026
Standard: ANSI/ASME B94.11M

Letter Drill · O · nominal diameter

0.3160 in

Millimetre: 8.026mm

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

O Drill Bit Size in Inches and Millimetres

The nominal diameter of a O 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.

O drill bit — decimal inch and millimetre diameter
Property	Value
Diameter, decimal inch	0.3160 in
Diameter, millimetre	8.026 mm
Cross-sectional area	0.07843 in²

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

Threads a O Drill Bit Taps

Every thread in this reference whose tap drill is the O 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 O drill bit, by thread engagement
Thread	Series	Thread % asked for	Thread % this drill leaves
3/8"-16 UNC	Unified Coarse (UNC)	70%	72.7

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

O Drill Bit Equivalents in Every Other Drill Series

The closest size to the O 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 O drill bit
Property	Property	Diameter		Property
Drill	Series	in	mm	Difference, in
#1	Number	0.2280	5.791	−0.0880
5/16"	Fractional	0.3125	7.938	−0.0035
8 mm	Metric	0.3150	8.000	−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 O 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 O drill bit, by diameter
Property	Property	Diameter
Drill	Series	in	mm
5/16"	Fractional	0.3125	7.938
8 mm	Metric	0.3150	8.000
O	Letter	0.3160	8.026
8.1 mm	Metric	0.3189	8.100
8.2 mm	Metric	0.3228	8.200

Sources and Method for the O Drill Bit Size

Drill diametercompiled

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

Compiled from independent public sources that agree cell-for-cell.

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.
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 O Drill Bit Reference

Citation

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

BibTeX

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

Permanent URL

https://shopmath.org/drills/letter/o

Common Questions About the O Drill Bit

What size is a O drill bit?: A O drill bit is 0.3160 in in diameter, which is 8.026 mm.
What is the metric equivalent of a O drill bit?: The closest metric drill to a O drill bit (8.026 mm) is 8 mm, which is 8.000 mm — 0.0010 in smaller.
What fraction is a O drill bit closest to?: A O drill bit (0.3160 in) sits closest to the 5/16" fractional drill at 0.3125 in, a difference of 0.0035 in.

All letter drill sizes · Complete drill size chart · Every drill series