---
type: "drill"
designation: "8 mm"
series: "metric"
series_name: "Metric Drill Sizes"
diameter_in: 0.314961
diameter_mm: 8
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/metric/8mm
---

# 8 mm Drill Bit Size — 0.3150 in / 8.000 mm

A 8 mm drill bit is 0.3150 in (8.000 mm) in diameter. It belongs to the metric series of standard twist drills.

- **Designation**: 8 mm
- **Series**: Metric
- **Diameter**: 0.3150 in (8.000 mm)

## 8 mm Drill Diameter

| Property | Value |
| --- | --- |
| Designation | 8 mm |
| Series | Metric |
| Diameter (in) | 0.3150 |
| Diameter (mm) | 8.000 |

## 8 mm Drill Equivalents in the Other Size Series

The nearest standard drill to 8 mm 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 | -0.0870 in |
| Letter | [O](https://shopmath.org/drills/letter/o.md) | 0.3160 | 8.026 | +0.0010 in |
| Fractional | [5/16](https://shopmath.org/drills/fractional/5-16.md) | 0.3125 | 7.938 | -0.0025 in |

## Sources and Method for the 8 mm 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**: computed
- **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 8 mm Drill Bit

### What size is a 8 mm drill bit size?

A 8 mm drill bit is 0.3150 in (8.000 mm) in diameter.

### What fraction is a 8 mm drill bit?

8 mm is 0.3150 in (8.000 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): [5/16](https://shopmath.org/drills/fractional/5-16.md) — 0.3125 in (7.938 mm)
- Next size up (any series): [O](https://shopmath.org/drills/letter/o.md) — 0.3160 in (8.026 mm)
- Previous in the metric series: [7.9 mm](https://shopmath.org/drills/metric/7-9mm.md)
- Next in the metric series: [8.1 mm](https://shopmath.org/drills/metric/8-1mm.md)
- All metric sizes: [Metric drill sizes](https://shopmath.org/drills/metric.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)

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*Source: [ShopMath](https://shopmath.org/drills/metric/8mm) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*