---
type: "drill-series"
series: "letter"
name: "Letter Drill Sizes"
size_count: 26
smallest: "A"
largest: "Z"
range_in: "0.2340–0.4130"
range_mm: "5.944–10.490"
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/letter
---

# Letter Drill Sizes — All 26 Sizes in Inches and Millimetres

The letter drill sizes series runs from A at 0.2340 in (5.944 mm) to Z at 0.4130 in (10.490 mm), 26 sizes in all. Letter-gauge twist drills, A (smallest) through Z (largest), 0.234 to 0.413 in.

- **Series**: Letter Drill Sizes
- **Sizes**: 26
- **Smallest**: A — 0.2340 in
- **Largest**: Z — 0.4130 in
- **Standard**: ANSI/ASME B94.11M; BS 328

## Every Letter Drill Size

Each size links to its page with the nearest equivalent in the other three series and the threads it taps.

| Drill | Diameter (in) | Diameter (mm) | Fraction | Taps |
| --- | --- | --- | --- | --- |
| [A](https://shopmath.org/drills/letter/a.md) | 0.2340 | 5.944 |  |  |
| [B](https://shopmath.org/drills/letter/b.md) | 0.2380 | 6.045 |  |  |
| [C](https://shopmath.org/drills/letter/c.md) | 0.2420 | 6.147 |  |  |
| [D](https://shopmath.org/drills/letter/d.md) | 0.2460 | 6.248 |  |  |
| [E](https://shopmath.org/drills/letter/e.md) | 0.2500 | 6.350 |  |  |
| [F](https://shopmath.org/drills/letter/f.md) | 0.2570 | 6.528 |  | 5/16-18 UNC |
| [G](https://shopmath.org/drills/letter/g.md) | 0.2610 | 6.629 |  |  |
| [H](https://shopmath.org/drills/letter/h.md) | 0.2660 | 6.756 |  |  |
| [I](https://shopmath.org/drills/letter/i.md) | 0.2720 | 6.909 |  | 5/16-24 UNF |
| [J](https://shopmath.org/drills/letter/j.md) | 0.2770 | 7.036 |  |  |
| [K](https://shopmath.org/drills/letter/k.md) | 0.2810 | 7.137 |  |  |
| [L](https://shopmath.org/drills/letter/l.md) | 0.2900 | 7.366 |  |  |
| [M](https://shopmath.org/drills/letter/m.md) | 0.2950 | 7.493 |  |  |
| [N](https://shopmath.org/drills/letter/n.md) | 0.3020 | 7.671 |  |  |
| [O](https://shopmath.org/drills/letter/o.md) | 0.3160 | 8.026 |  |  |
| [P](https://shopmath.org/drills/letter/p.md) | 0.3230 | 8.204 |  |  |
| [Q](https://shopmath.org/drills/letter/q.md) | 0.3320 | 8.433 |  | 3/8-24 UNF |
| [R](https://shopmath.org/drills/letter/r.md) | 0.3390 | 8.611 |  |  |
| [S](https://shopmath.org/drills/letter/s.md) | 0.3480 | 8.839 |  |  |
| [T](https://shopmath.org/drills/letter/t.md) | 0.3580 | 9.093 |  |  |
| [U](https://shopmath.org/drills/letter/u.md) | 0.3680 | 9.347 |  | 7/16-14 UNC |
| [V](https://shopmath.org/drills/letter/v.md) | 0.3770 | 9.576 |  |  |
| [W](https://shopmath.org/drills/letter/w.md) | 0.3860 | 9.804 |  |  |
| [X](https://shopmath.org/drills/letter/x.md) | 0.3970 | 10.084 |  |  |
| [Y](https://shopmath.org/drills/letter/y.md) | 0.4040 | 10.262 |  | 7/16-28 UNEF |
| [Z](https://shopmath.org/drills/letter/z.md) | 0.4130 | 10.490 |  |  |

_Diameters are nominal. The taps column lists the threads this drill is the recommended tap drill for at the default engagement._

## Sources and Method for the Letter Drill Series

### Letter drill sizes

- **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.
- 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

## Related pages

- All drill series: [Drill bit size reference](https://shopmath.org/drills.md)
- Every series interleaved 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) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*