Inch-system twist drills come in three overlapping series. The number series (#1–#80) covers the smallest sizes, where a larger number means a smaller hole. The letter series (A–Z) picks up just above #1, with Z the largest, and the fractional series steps in even 1/64-inch increments from 1/64 up to 1/2 inch and beyond. Number and letter sizes are defined values from the ANSI/ASME B94.11M standard — not fractions you can derive — so this lookup table is the source of truth. To open a hole slightly, pick the next size up in decimal inches (the next row down in this chart) regardless of which series it belongs to: for example, the size just over a #7 (0.2010 in) is a #6 (0.2040 in), then a #5, then a letter A (0.2340 in).
| Size | Type | Decimal (in) | mm |
|---|---|---|---|
| #80 | Number | 0.0135 | 0.343 |
| #79 | Number | 0.0145 | 0.368 |
| 1/64" | Fraction | 0.0156 | 0.397 |
| #78 | Number | 0.0160 | 0.406 |
| #77 | Number | 0.0180 | 0.457 |
| #76 | Number | 0.0200 | 0.508 |
| #75 | Number | 0.0210 | 0.533 |
| #74 | Number | 0.0225 | 0.571 |
| #73 | Number | 0.0240 | 0.610 |
| #72 | Number | 0.0250 | 0.635 |
| #71 | Number | 0.0260 | 0.660 |
| #70 | Number | 0.0280 | 0.711 |
| #69 | Number | 0.0292 | 0.742 |
| #68 | Number | 0.0310 | 0.787 |
| 1/32" | Fraction | 0.0313 | 0.794 |
| #67 | Number | 0.0320 | 0.813 |
| #66 | Number | 0.0330 | 0.838 |
| #65 | Number | 0.0350 | 0.889 |
| #64 | Number | 0.0360 | 0.914 |
| #63 | Number | 0.0370 | 0.940 |
| #62 | Number | 0.0380 | 0.965 |
| #61 | Number | 0.0390 | 0.991 |
| #60 | Number | 0.0400 | 1.016 |
| #59 | Number | 0.0410 | 1.041 |
| #58 | Number | 0.0420 | 1.067 |
| #57 | Number | 0.0430 | 1.092 |
| #56 | Number | 0.0465 | 1.181 |
| 3/64" | Fraction | 0.0469 | 1.191 |
| #55 | Number | 0.0520 | 1.321 |
| #54 | Number | 0.0550 | 1.397 |
| #53 | Number | 0.0595 | 1.511 |
| 1/16" | Fraction | 0.0625 | 1.587 |
| #52 | Number | 0.0635 | 1.613 |
| #51 | Number | 0.0670 | 1.702 |
| #50 | Number | 0.0700 | 1.778 |
| #49 | Number | 0.0730 | 1.854 |
| #48 | Number | 0.0760 | 1.930 |
| 5/64" | Fraction | 0.0781 | 1.984 |
| #47 | Number | 0.0785 | 1.994 |
| #46 | Number | 0.0810 | 2.057 |
| #45 | Number | 0.0820 | 2.083 |
| #44 | Number | 0.0860 | 2.184 |
| #43 | Number | 0.0890 | 2.261 |
| #42 | Number | 0.0935 | 2.375 |
| 3/32" | Fraction | 0.0938 | 2.381 |
| #41 | Number | 0.0960 | 2.438 |
| #40 | Number | 0.0980 | 2.489 |
| #39 | Number | 0.0995 | 2.527 |
| #38 | Number | 0.1015 | 2.578 |
| #37 | Number | 0.1040 | 2.642 |
| #36 | Number | 0.1065 | 2.705 |
| 7/64" | Fraction | 0.1094 | 2.778 |
| #35 | Number | 0.1100 | 2.794 |
| #34 | Number | 0.1110 | 2.819 |
| #33 | Number | 0.1130 | 2.870 |
| #32 | Number | 0.1160 | 2.946 |
| #31 | Number | 0.1200 | 3.048 |
| 1/8" | Fraction | 0.1250 | 3.175 |
| #30 | Number | 0.1285 | 3.264 |
| #29 | Number | 0.1360 | 3.454 |
| #28 | Number | 0.1405 | 3.569 |
| 9/64" | Fraction | 0.1406 | 3.572 |
| #27 | Number | 0.1440 | 3.658 |
| #26 | Number | 0.1470 | 3.734 |
| #25 | Number | 0.1495 | 3.797 |
| #24 | Number | 0.1520 | 3.861 |
| #23 | Number | 0.1540 | 3.912 |
| 5/32" | Fraction | 0.1563 | 3.969 |
| #22 | Number | 0.1570 | 3.988 |
| #21 | Number | 0.1590 | 4.039 |
| #20 | Number | 0.1610 | 4.089 |
| #19 | Number | 0.1660 | 4.216 |
| #18 | Number | 0.1695 | 4.305 |
| 11/64" | Fraction | 0.1719 | 4.366 |
| #17 | Number | 0.1730 | 4.394 |
| #16 | Number | 0.1770 | 4.496 |
| #15 | Number | 0.1800 | 4.572 |
| #14 | Number | 0.1820 | 4.623 |
| #13 | Number | 0.1850 | 4.699 |
| 3/16" | Fraction | 0.1875 | 4.762 |
| #12 | Number | 0.1890 | 4.801 |
| #11 | Number | 0.1910 | 4.851 |
| #10 | Number | 0.1935 | 4.915 |
| #9 | Number | 0.1960 | 4.978 |
| #8 | Number | 0.1990 | 5.055 |
| #7 | Number | 0.2010 | 5.105 |
| 13/64" | Fraction | 0.2031 | 5.159 |
| #6 | Number | 0.2040 | 5.182 |
| #5 | Number | 0.2055 | 5.220 |
| #4 | Number | 0.2090 | 5.309 |
| #3 | Number | 0.2130 | 5.410 |
| 7/32" | Fraction | 0.2188 | 5.556 |
| #2 | Number | 0.2210 | 5.613 |
| #1 | Number | 0.2280 | 5.791 |
| A | Letter | 0.2340 | 5.944 |
| 15/64" | Fraction | 0.2344 | 5.953 |
| B | Letter | 0.2380 | 6.045 |
| C | Letter | 0.2420 | 6.147 |
| D | Letter | 0.2460 | 6.248 |
| E | Letter | 0.2500 | 6.350 |
| 1/4" | Fraction | 0.2500 | 6.350 |
| F | Letter | 0.2570 | 6.528 |
| G | Letter | 0.2610 | 6.629 |
| 17/64" | Fraction | 0.2656 | 6.747 |
| H | Letter | 0.2660 | 6.756 |
| I | Letter | 0.2720 | 6.909 |
| J | Letter | 0.2770 | 7.036 |
| K | Letter | 0.2810 | 7.137 |
| 9/32" | Fraction | 0.2813 | 7.144 |
| L | Letter | 0.2900 | 7.366 |
| M | Letter | 0.2950 | 7.493 |
| 19/64" | Fraction | 0.2969 | 7.541 |
| N | Letter | 0.3020 | 7.671 |
| 5/16" | Fraction | 0.3125 | 7.938 |
| O | Letter | 0.3160 | 8.026 |
| P | Letter | 0.3230 | 8.204 |
| 21/64" | Fraction | 0.3281 | 8.334 |
| Q | Letter | 0.3320 | 8.433 |
| R | Letter | 0.3390 | 8.611 |
| 11/32" | Fraction | 0.3438 | 8.731 |
| S | Letter | 0.3480 | 8.839 |
| T | Letter | 0.3580 | 9.093 |
| 23/64" | Fraction | 0.3594 | 9.128 |
| U | Letter | 0.3680 | 9.347 |
| 3/8" | Fraction | 0.3750 | 9.525 |
| V | Letter | 0.3770 | 9.576 |
| W | Letter | 0.3860 | 9.804 |
| 25/64" | Fraction | 0.3906 | 9.922 |
| X | Letter | 0.3970 | 10.084 |
| Y | Letter | 0.4040 | 10.262 |
| 13/32" | Fraction | 0.4063 | 10.319 |
| Z | Letter | 0.4130 | 10.490 |
| 27/64" | Fraction | 0.4219 | 10.716 |
| 7/16" | Fraction | 0.4375 | 11.112 |
| 29/64" | Fraction | 0.4531 | 11.509 |
| 15/32" | Fraction | 0.4688 | 11.906 |
| 31/64" | Fraction | 0.4844 | 12.303 |
| 1/2" | Fraction | 0.5000 | 12.700 |
Source: ANSI/ASME B94.11M (twist-drill sizes). Verify against a current machinist's reference.
Related tools