Reference & Charts
Drill Size Chart
Decimal-inch and millimetre equivalents for the standard drill series — fractional (by 64ths), number (#1–#40), and letter (A–Z) — so you can convert any drill callout to a size you can measure or order.
Last verified: 2026-07-09
Drill sizes come in three overlapping series: fractional (in 1/64" steps), number gauge (#1 largest to #80 smallest), and letter gauge (A smallest to Z largest, sitting just above 1/4"). These charts give the decimal-inch and millimetre size for the common range of each so you can compare, measure, or convert a callout. Millimetre values are the inch size × 25.4.
Fractional drills
| Fraction | Decimal (in) | mm |
|---|---|---|
| 1/16" | 0.0625 | 1.59 |
| 3/32" | 0.0938 | 2.38 |
| 1/8" | 0.125 | 3.18 |
| 5/32" | 0.1563 | 3.97 |
| 3/16" | 0.1875 | 4.76 |
| 7/32" | 0.2188 | 5.56 |
| 1/4" | 0.25 | 6.35 |
| 9/32" | 0.2813 | 7.14 |
| 5/16" | 0.3125 | 7.94 |
| 11/32" | 0.3438 | 8.73 |
| 3/8" | 0.375 | 9.53 |
| 13/32" | 0.4063 | 10.32 |
| 7/16" | 0.4375 | 11.11 |
| 15/32" | 0.4688 | 11.91 |
| 1/2" | 0.5 | 12.7 |
Number drills
| Number | Decimal (in) | mm |
|---|---|---|
| #1 | 0.228 | 5.79 |
| #2 | 0.221 | 5.61 |
| #3 | 0.213 | 5.41 |
| #4 | 0.209 | 5.31 |
| #5 | 0.2055 | 5.22 |
| #6 | 0.204 | 5.18 |
| #7 | 0.201 | 5.11 |
| #8 | 0.199 | 5.05 |
| #9 | 0.196 | 4.98 |
| #10 | 0.1935 | 4.91 |
| #12 | 0.189 | 4.8 |
| #16 | 0.177 | 4.5 |
| #20 | 0.161 | 4.09 |
| #21 | 0.159 | 4.04 |
| #25 | 0.1495 | 3.8 |
| #29 | 0.136 | 3.45 |
| #30 | 0.1285 | 3.26 |
| #31 | 0.12 | 3.05 |
| #36 | 0.1065 | 2.71 |
| #40 | 0.098 | 2.49 |
Letter drills
| Letter | Decimal (in) | mm |
|---|---|---|
| A | 0.234 | 5.94 |
| B | 0.238 | 6.05 |
| C | 0.242 | 6.15 |
| D | 0.246 | 6.25 |
| E | 0.25 | 6.35 |
| F | 0.257 | 6.53 |
| G | 0.261 | 6.63 |
| H | 0.266 | 6.76 |
| I | 0.272 | 6.91 |
| J | 0.277 | 7.04 |
| K | 0.281 | 7.14 |
| L | 0.29 | 7.37 |
| M | 0.295 | 7.49 |
| N | 0.302 | 7.67 |
| O | 0.316 | 8.03 |
| P | 0.323 | 8.2 |
| Q | 0.332 | 8.43 |
| R | 0.339 | 8.61 |
| S | 0.348 | 8.84 |
| T | 0.358 | 9.09 |
| U | 0.368 | 9.35 |
| V | 0.377 | 9.58 |
| W | 0.386 | 9.8 |
| X | 0.397 | 10.08 |
| Y | 0.404 | 10.26 |
| Z | 0.413 | 10.49 |
Frequently asked
- How do number and letter drills relate to fractions?
- They fill the gaps between fractional sizes. Number drills run smallest around #80 up to #1 (0.228"); letter drills pick up just below 1/4" at A (0.234") and run up to Z (0.413"). Use them when a fractional drill is too coarse a step for the hole you need.
- Why are the millimetre values not round numbers?
- These are inch-defined drills, so the metric column is just the conversion (inch × 25.4). If you need true metric drills (3.0, 3.2, 3.5 mm…), those are a separate series.
- Which drill do I use before tapping?
- That depends on the thread — see the Tap Drill Chart, which lists the correct tap drill for each UNC/UNF size at ~75% thread engagement.
Sources
- ANSI/ASME B94.11M — Twist Drills (fractional, number, and letter drill series)
- Standard machinist decimal-equivalent drill tables