---
type: "hub"
name: "Drill bit size reference"
series_count: 4
size_count: 377
series: ["fractional", "letter", "metric", "number"]
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills
---

# Drill Bit Size Reference — Number, Letter, Fractional and Metric

377 standard twist drill sizes across four numbering series, each with its diameter in inch and millimetre, the nearest equivalent in every other series, and the threads it taps. Number, letter, fractional and metric sizes interleave in one continuous size order — that is the lookup this reference exists to make trivial.

## Standard Drill Size Series

| Series | Sizes | Smallest | Largest | Range (in) | Range (mm) | What it is |
| --- | --- | --- | --- | --- | --- | --- |
| [Fractional Drill Sizes](https://shopmath.org/drills/fractional.md) | 64 | [1/64](https://shopmath.org/drills/fractional/1-64.md) | [1](https://shopmath.org/drills/fractional/1.md) | 0.0156 – 1.0000 in | 0.397 – 25.400 mm | Inch fractional drills in 1/64 in steps from 1/64 to 1 in. |
| [Letter Drill Sizes](https://shopmath.org/drills/letter.md) | 26 | [A](https://shopmath.org/drills/letter/a.md) | [Z](https://shopmath.org/drills/letter/z.md) | 0.2340 – 0.4130 in | 5.944 – 10.490 mm | Letter-gauge twist drills, A (smallest) through Z (largest), 0.234 to 0.413 in. |
| [Metric Drill Sizes](https://shopmath.org/drills/metric.md) | 227 | [0.2 mm](https://shopmath.org/drills/metric/0-2mm.md) | [25 mm](https://shopmath.org/drills/metric/25mm.md) | 0.0079 – 0.9843 in | 0.200 – 25.000 mm | Metric twist drills, 0.2 mm to 25 mm on the BS 328 increment pattern. |
| [Number Drill Sizes](https://shopmath.org/drills/number.md) | 60 | [#60](https://shopmath.org/drills/number/60.md) | [#1](https://shopmath.org/drills/number/1.md) | 0.0400 – 0.2280 in | 1.016 – 5.791 mm | Gauge-numbered twist drills, #1 (largest) through #80 (smallest). |

_A series appears here only when it carries at least three verified sizes._

## Sources and Method for the Drill Index

### Standard drill index — number, letter, fractional, metric

- **Source**: bd_warehouse drill_sizes.csv (Apache-2.0) + Wikipedia Drill bit sizes (CC BY-SA 4.0); fractional and metric computed from ANSI B94.11M / BS 328 increment patterns
- **Method**: computed+compiled
- **Verified against**: drills: 31 checks passed across 1 harness(es)

### 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

- 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)
- Threads: [Screw thread reference](https://shopmath.org/threads.md)

---

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