---
type: "drill"
designation: "O"
series: "letter"
series_name: "Letter Drill Sizes"
diameter_in: 0.316
diameter_mm: 8.0264
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/letter/o
---

# O Drill Bit Size — 0.3160 in / 8.026 mm

A O drill bit is 0.3160 in (8.026 mm) in diameter. It belongs to the letter series of standard twist drills.

- **Designation**: O
- **Series**: Letter
- **Diameter**: 0.3160 in (8.026 mm)

## O Drill Diameter

| Property | Value |
| --- | --- |
| Designation | O |
| Series | Letter |
| Diameter (in) | 0.3160 |
| Diameter (mm) | 8.026 |

## O Drill Equivalents in the Other Size Series

The nearest standard drill to O in each of the other three series. Nothing here is an exact match unless the difference column reads 0.0000 in.

| Series | Nearest drill | Diameter (in) | Diameter (mm) | Difference |
| --- | --- | --- | --- | --- |
| Number | [#1](https://shopmath.org/drills/number/1.md) | 0.2280 | 5.791 | -0.0880 in |
| Fractional | [5/16](https://shopmath.org/drills/fractional/5-16.md) | 0.3125 | 7.938 | -0.0035 in |
| Metric | [8 mm](https://shopmath.org/drills/metric/8mm.md) | 0.3150 | 8.000 | -0.0010 in |

## Sources and Method for the O Drill Size

### Drill index

- **Source**: 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.
- **Method**: compiled
- **Verified against**: Wikipedia Drill bit sizes (CC BY-SA 4.0); 31/31 checks passed

### Assumptions

- Drill diameters are nominal. A worn or run-out drill cuts oversize; measure the hole, not the shank, when the fit matters.
- 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

## Common Questions About the O Drill Bit

### What size is a O drill bit size?

A O drill bit is 0.3160 in (8.026 mm) in diameter.

### What is the metric equivalent of a O drill bit?

The closest metric drill to O (0.3160 in (8.026 mm)) is 8 mm at 0.3150 in (8.000 mm) — a difference of 0.0010 in.

### What fraction is a O drill bit?

O is 0.3160 in (8.026 mm), which is not itself a fractional size. The closest fractional drill is 5/16 at 0.3125 in (7.938 mm).

## Related pages

- Next size down (any series): [8 mm](https://shopmath.org/drills/metric/8mm.md) — 0.3150 in (8.000 mm)
- Next size up (any series): [8.1 mm](https://shopmath.org/drills/metric/8-1mm.md) — 0.3189 in (8.100 mm)
- Previous in the letter series: [N](https://shopmath.org/drills/letter/n.md)
- Next in the letter series: [P](https://shopmath.org/drills/letter/p.md)
- All letter sizes: [Letter drill sizes](https://shopmath.org/drills/letter.md)
- All drill sizes: [Drill bit size reference](https://shopmath.org/drills.md)
- Complete index by diameter: [Drill size chart](https://shopmath.org/charts/drill-sizes.md)
- Tap drills by thread: [Tap drill chart](https://shopmath.org/charts/tap-drill.md)

---

*Source: [ShopMath](https://shopmath.org/drills/letter/o) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*