---
type: "drill"
designation: "21/64"
series: "fractional"
series_name: "Fractional Drill Sizes"
diameter_in: 0.328125
diameter_mm: 8.334375
fraction_label: "21/64"
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/fractional/21-64
---

# 21/64 Drill Bit Size — 0.3281 in / 8.334 mm

A 21/64 drill bit is 0.3281 in (8.334 mm) in diameter. It belongs to the fractional series of standard twist drills.

- **Designation**: 21/64
- **Series**: Fractional
- **Diameter**: 0.3281 in (8.334 mm)
- **Fraction**: 21/64

## 21/64 Drill Diameter

| Property | Value |
| --- | --- |
| Designation | 21/64 |
| Series | Fractional |
| Diameter (in) | 0.3281 |
| Diameter (mm) | 8.334 |
| Fractional call-out | 21/64 |

## 21/64 Drill Equivalents in the Other Size Series

The nearest standard drill to 21/64 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 | -0.1001 in |
| Letter | [Q](https://shopmath.org/drills/letter/q.md) | 0.3320 | 8.433 | +0.0039 in |
| Metric | [8.3 mm](https://shopmath.org/drills/metric/8-3mm.md) | 0.3268 | 8.300 | -0.0014 in |

## Sources and Method for the 21/64 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 21/64 Drill Bit

### What size is a 21/64 drill bit size?

A 21/64 drill bit is 0.3281 in (8.334 mm) in diameter. The nearest fractional call-out is 21/64.

### What is the metric equivalent of a 21/64 drill bit?

The closest metric drill to 21/64 (0.3281 in (8.334 mm)) is 8.3 mm at 0.3268 in (8.300 mm) — a difference of 0.0014 in.

## Related pages

- Next size down (any series): [8.3 mm](https://shopmath.org/drills/metric/8-3mm.md) — 0.3268 in (8.300 mm)
- Next size up (any series): [8.4 mm](https://shopmath.org/drills/metric/8-4mm.md) — 0.3307 in (8.400 mm)
- Previous in the fractional series: [5/16](https://shopmath.org/drills/fractional/5-16.md)
- Next in the fractional series: [11/32](https://shopmath.org/drills/fractional/11-32.md)
- All fractional sizes: [Fractional drill sizes](https://shopmath.org/drills/fractional.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/fractional/21-64) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*