Drill series · Letter
Letter Drill Sizes — Decimal Inch and Millimetre Chart
https://shopmath.org/drills/letter · data verified August 1, 2026
The letter drill sizes run from A at 0.2340 in (5.944 mm) to Z at 0.4130 in (10.490 mm), and 5 of the 26 sizes are the default tap drill for at least one standard thread. Letter-gauge twist drills, A (smallest) through Z (largest), 0.234 to 0.413 in.
- Sizes
- 26
- Smallest
- A
- Largest
- Z
- Standard
- ANSI/ASME B94.11M
Every Letter Drill Bit Size, in Inches and Millimetres
26 sizes, ordered smallest to largest. Every row links to that drill's own page, with its equivalents in the other three indexes and the threads it taps.
| Property | Diameter | Property | |
|---|---|---|---|
| Drill | in | mm | Tap drill for |
| A | 0.2340 | 5.944 | — |
| B | 0.2380 | 6.045 | — |
| C | 0.2420 | 6.147 | — |
| D | 0.2460 | 6.248 | — |
| E | 0.2500 | 6.350 | — |
| F | 0.2570 | 6.528 | 5/16"-18 UNC |
| G | 0.2610 | 6.629 | — |
| H | 0.2660 | 6.756 | — |
| I | 0.2720 | 6.909 | 5/16"-24 UNF |
| J | 0.2770 | 7.036 | — |
| K | 0.2810 | 7.137 | — |
| L | 0.2900 | 7.366 | — |
| M | 0.2950 | 7.493 | — |
| N | 0.3020 | 7.671 | — |
| O | 0.3160 | 8.026 | — |
| P | 0.3230 | 8.204 | — |
| Q | 0.3320 | 8.433 | 3/8"-24 UNF |
| R | 0.3390 | 8.611 | — |
| S | 0.3480 | 8.839 | — |
| T | 0.3580 | 9.093 | — |
| U | 0.3680 | 9.347 | 7/16"-14 UNC |
| V | 0.3770 | 9.576 | — |
| W | 0.3860 | 9.804 | — |
| X | 0.3970 | 10.084 | — |
| Y | 0.4040 | 10.262 | 7/16"-28 UNEF |
| Z | 0.4130 | 10.490 | — |
Diameters are nominal. The tap-drill column shows the threads whose shop-default tap drill is that size; a drill with no entry there is still a standard size, it is simply not the default drill for any thread in this reference.
Sources and Method for the Letter Drill Bit Chart
- Letter drill diameterscompiled
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.
Verified against Wikipedia Drill bit sizes (CC BY-SA 4.0); 31/31 checks passed
Compiled from independent public sources that agree cell-for-cell.
- Assumptions
- Diameters are nominal. Manufacturing tolerance, wear and run-out all cut 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
How to Cite This Letter Drill Bit Chart
Citation
ShopMath. "Letter Drill Sizes — Decimal Inch and Millimetre Chart." ShopMath, verified August 1, 2026, https://shopmath.org/drills/letter
BibTeX
@misc{shopmath-drills-letter,
title = {Letter Drill Sizes — Decimal Inch and Millimetre Chart},
author = {{ShopMath}},
year = {2026},
howpublished = {\url{https://shopmath.org/drills/letter}},
note = {Data verified 2026-08-01}
}Permanent URL
https://shopmath.org/drills/letter