---
type: "drill"
designation: "Q"
series: "letter"
series_name: "Letter Drill Sizes"
diameter_in: 0.332
diameter_mm: 8.4328
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/letter/q
---

# Q Drill Bit Size — 0.3320 in / 8.433 mm

A Q drill bit is 0.3320 in (8.433 mm) in diameter. It belongs to the letter series of standard twist drills. It is the recommended tap drill for 3/8"-24 UNF.

- **Designation**: Q
- **Series**: Letter
- **Diameter**: 0.3320 in (8.433 mm)

## Q Drill Diameter

| Property | Value |
| --- | --- |
| Designation | Q |
| Series | Letter |
| Diameter (in) | 0.3320 |
| Diameter (mm) | 8.433 |

## Q Drill Equivalents in the Other Size Series

The nearest standard drill to Q 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 | -0.1040 in |
| Fractional | [21/64](https://shopmath.org/drills/fractional/21-64.md) | 0.3281 | 8.334 | -0.0039 in |
| Metric | [8.4 mm](https://shopmath.org/drills/metric/8-4mm.md) | 0.3307 | 8.400 | -0.0013 in |

## Threads Tapped With a Q Drill

| Thread | Pitch | Thread engagement (%) |
| --- | --- | --- |
| [3/8"-24 UNF](https://shopmath.org/threads/unf/3-8-24.md) | 24 TPI | 79.4 |

_Engagement is what the real drill leaves with a cut tap, not the requested target._

## Sources and Method for the Q 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 Q Drill Bit

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

A Q drill bit is 0.3320 in (8.433 mm) in diameter.

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

The closest metric drill to Q (0.3320 in (8.433 mm)) is 8.4 mm at 0.3307 in (8.400 mm) — a difference of 0.0013 in.

### What fraction is a Q drill bit?

Q is 0.3320 in (8.433 mm), which is not itself a fractional size. The closest fractional drill is 21/64 at 0.3281 in (8.334 mm).

### What thread does a Q drill bit tap?

A Q drill is the recommended tap drill for 3/8"-24 UNF. That leaves the thread engagement shown in the table above with a cut tap.

## Related pages

- Next size down (any series): [8.4 mm](https://shopmath.org/drills/metric/8-4mm.md) — 0.3307 in (8.400 mm)
- Next size up (any series): [8.5 mm](https://shopmath.org/drills/metric/8-5mm.md) — 0.3346 in (8.500 mm)
- Previous in the letter series: [P](https://shopmath.org/drills/letter/p.md)
- Next in the letter series: [R](https://shopmath.org/drills/letter/r.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/q) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*