Drill Bit Size Chart — Fractional, Number, Letter & Metric Decimal Equivalents
ANSI/ASME B94.11M drill sizes: number drills #80–#1, letter drills A–Z, and fractional bits 1/64″ to 1/2″ with decimal-inch and millimetre equivalents.
Updated August 18, 2026
US twist drills come in three interleaved inch-system families. Fractional drills step in 64ths of an inch and are exact by definition. Number (wire-gauge) drills #80–#1 fill the gaps below and between the small fractions — and the numbering runs backwards, so #80 (0.0135 in) is the smallest and #1 (0.228 in) the largest. Letter drills A–Z continue where the numbers leave off, from A (0.234 in) just above #1 up to Z (0.413 in). The gauge diameters are not on any arithmetic progression; they are the standardized decimal equivalents published in ANSI/ASME B94.11M and reproduced in Machinery’s Handbook.
The three families exist because fractional steps alone are too coarse for small holes: between 1/16″ and 5/64″ there is a 0.0156 in jump, but five number drills (#52–#48) land inside it. That density is what makes tap-drill charts work — the standard tap drill for a 1/4-20 UNC thread is #7 (0.201 in), not a fraction. The millimetre column below is computed as 25.4 × the decimal-inch value, and the whole chart is generated from the same drill table the tap drill size calculator searches, so the two can never drift apart.
Number (Wire Gauge) Drills #80–#1
| Drill size | Decimal (in) | Ø (mm) |
|---|---|---|
| #80 | 0.0135 | 0.343 |
| #79 | 0.0145 | 0.368 |
| #78 | 0.0160 | 0.406 |
| #77 | 0.0180 | 0.457 |
| #76 | 0.0200 | 0.508 |
| #75 | 0.0210 | 0.533 |
| #74 | 0.0225 | 0.571 |
| #73 | 0.0240 | 0.610 |
| #72 | 0.0250 | 0.635 |
| #71 | 0.0260 | 0.660 |
| #70 | 0.0280 | 0.711 |
| #69 | 0.0292 | 0.742 |
| #68 | 0.0310 | 0.787 |
| #67 | 0.0320 | 0.813 |
| #66 | 0.0330 | 0.838 |
| #65 | 0.0350 | 0.889 |
| #64 | 0.0360 | 0.914 |
| #63 | 0.0370 | 0.940 |
| #62 | 0.0380 | 0.965 |
| #61 | 0.0390 | 0.991 |
| #60 | 0.0400 | 1.016 |
| #59 | 0.0410 | 1.041 |
| #58 | 0.0420 | 1.067 |
| #57 | 0.0430 | 1.092 |
| #56 | 0.0465 | 1.181 |
| #55 | 0.0520 | 1.321 |
| #54 | 0.0550 | 1.397 |
| #53 | 0.0595 | 1.511 |
| #52 | 0.0635 | 1.613 |
| #51 | 0.0670 | 1.702 |
| #50 | 0.0700 | 1.778 |
| #49 | 0.0730 | 1.854 |
| #48 | 0.0760 | 1.930 |
| #47 | 0.0785 | 1.994 |
| #46 | 0.0810 | 2.057 |
| #45 | 0.0820 | 2.083 |
| #44 | 0.0860 | 2.184 |
| #43 | 0.0890 | 2.261 |
| #42 | 0.0935 | 2.375 |
| #41 | 0.0960 | 2.438 |
| #40 | 0.0980 | 2.489 |
| #39 | 0.0995 | 2.527 |
| #38 | 0.1015 | 2.578 |
| #37 | 0.1040 | 2.642 |
| #36 | 0.1065 | 2.705 |
| #35 | 0.1100 | 2.794 |
| #34 | 0.1110 | 2.819 |
| #33 | 0.1130 | 2.870 |
| #32 | 0.1160 | 2.946 |
| #31 | 0.1200 | 3.048 |
| #30 | 0.1285 | 3.264 |
| #29 | 0.1360 | 3.454 |
| #28 | 0.1405 | 3.569 |
| #27 | 0.1440 | 3.658 |
| #26 | 0.1470 | 3.734 |
| #25 | 0.1495 | 3.797 |
| #24 | 0.1520 | 3.861 |
| #23 | 0.1540 | 3.912 |
| #22 | 0.1570 | 3.988 |
| #21 | 0.1590 | 4.039 |
| #20 | 0.1610 | 4.089 |
| #19 | 0.1660 | 4.216 |
| #18 | 0.1695 | 4.305 |
| #17 | 0.1730 | 4.394 |
| #16 | 0.1770 | 4.496 |
| #15 | 0.1800 | 4.572 |
| #14 | 0.1820 | 4.623 |
| #13 | 0.1850 | 4.699 |
| #12 | 0.1890 | 4.801 |
| #11 | 0.1910 | 4.851 |
| #10 | 0.1935 | 4.915 |
| #9 | 0.1960 | 4.978 |
| #8 | 0.1990 | 5.055 |
| #7 | 0.2010 | 5.105 |
| #6 | 0.2040 | 5.182 |
| #5 | 0.2055 | 5.220 |
| #4 | 0.2090 | 5.309 |
| #3 | 0.2130 | 5.410 |
| #2 | 0.2210 | 5.613 |
| #1 | 0.2280 | 5.791 |
ANSI/ASME B94.11M decimal equivalents. Numbering runs backwards: #80 is the smallest. mm computed as 25.4 × in.
Letter Drills A–Z
| Drill size | Decimal (in) | Ø (mm) |
|---|---|---|
| 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 |
| G | 0.2610 | 6.629 |
| H | 0.2660 | 6.756 |
| I | 0.2720 | 6.909 |
| 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 |
| R | 0.3390 | 8.611 |
| S | 0.3480 | 8.839 |
| T | 0.3580 | 9.093 |
| U | 0.3680 | 9.347 |
| V | 0.3770 | 9.576 |
| W | 0.3860 | 9.804 |
| X | 0.3970 | 10.084 |
| Y | 0.4040 | 10.262 |
| Z | 0.4130 | 10.490 |
Letter sizes continue above #1 (0.228 in). E is exactly 1/4″ (0.250 in). mm computed as 25.4 × in.
Fractional Drills 1/64″–1/2″ (by 64ths)
| Drill size | Decimal (in) | Ø (mm) |
|---|---|---|
| 1/64″ | 0.0156 | 0.397 |
| 1/32″ | 0.0313 | 0.794 |
| 3/64″ | 0.0469 | 1.191 |
| 1/16″ | 0.0625 | 1.587 |
| 5/64″ | 0.0781 | 1.984 |
| 3/32″ | 0.0938 | 2.381 |
| 7/64″ | 0.1094 | 2.778 |
| 1/8″ | 0.1250 | 3.175 |
| 9/64″ | 0.1406 | 3.572 |
| 5/32″ | 0.1563 | 3.969 |
| 11/64″ | 0.1719 | 4.366 |
| 3/16″ | 0.1875 | 4.762 |
| 13/64″ | 0.2031 | 5.159 |
| 7/32″ | 0.2188 | 5.556 |
| 15/64″ | 0.2344 | 5.953 |
| 1/4″ | 0.2500 | 6.350 |
| 17/64″ | 0.2656 | 6.747 |
| 9/32″ | 0.2813 | 7.144 |
| 19/64″ | 0.2969 | 7.541 |
| 5/16″ | 0.3125 | 7.938 |
| 21/64″ | 0.3281 | 8.334 |
| 11/32″ | 0.3438 | 8.731 |
| 23/64″ | 0.3594 | 9.128 |
| 3/8″ | 0.3750 | 9.525 |
| 25/64″ | 0.3906 | 9.922 |
| 13/32″ | 0.4063 | 10.319 |
| 27/64″ | 0.4219 | 10.716 |
| 7/16″ | 0.4375 | 11.112 |
| 29/64″ | 0.4531 | 11.509 |
| 15/32″ | 0.4688 | 11.906 |
| 31/64″ | 0.4844 | 12.303 |
| 1/2″ | 0.5000 | 12.700 |
Fractional sizes are exact n/64 arithmetic; the decimal column shows the conventional 4-decimal rounding. mm computed as 25.4 × in.
Sources & Further Reading
- ANSI/ASME B94.11M — straight-shank twist drill sizes; number (#80–#1) and letter (A–Z) gauge decimal equivalents, as reproduced in Machinery’s Handbook drill-size tables
- Victor Machinery and Rivcut published drill decimal-equivalent charts (independent cross-checks of every gauge value)
- Fractional sizes: exact n/64 in arithmetic; mm = 25.4 × in (computed, not transcribed)
Frequently Asked Questions
Why do number drill sizes run backwards?
Number drills inherit the wire-gauge convention: the gauge number counts drawing operations, so more draws mean thinner wire — and a smaller drill. #80 is the smallest at 0.0135 in and #1 the largest at 0.228 in; letter drills then continue upward from A (0.234 in) to Z (0.413 in).
Which drill is exactly 1/4 inch — and why does letter E exist?
Letter drill E is exactly 0.250 in, the same as the 1/4″ fractional drill. The letter series was defined as a continuous run, so it includes a few sizes that coincide with fractions (E = 1/4″). In practice suppliers sell it as 1/4″; charts list both names for the same diameter.
How do I convert a drill size to millimetres?
Multiply the decimal-inch equivalent by 25.4: a #7 drill is 0.201 × 25.4 = 5.105 mm, and a 3/8″ drill is 0.375 × 25.4 = 9.525 mm. Going the other way, the closest metric stock size is usually within a few hundredths of a millimetre — the tap drill size calculator reports the nearest cross-system substitute automatically.
What drill do I use before tapping a thread?
Look up the tap drill, not the thread size: the hole must sit at the thread’s minor diameter plus an engagement allowance. The Machinery’s Handbook formula is drill = major − 1.29904 × pitch × %engagement/100, and 75% engagement is the chart default — e.g. 1/4-20 UNC takes a #7 (0.201 in) drill. The tap drill size calculator applies the formula and picks the nearest standard drill from this chart.
Try the Calculators
Drill Bit Size Chart — Fractional, Number, Letter & Metric Decimal Equivalents — reuven.tools/reference/drill-bit-sizes — verified against: ANSI/ASME B94.11M — straight-shank twist drill sizes; number (#80–#1) and letter (A–Z) gauge decimal equivalents, as reproduced in Machinery’s Handbook drill-size tables; Victor Machinery and Rivcut published drill decimal-equivalent charts (independent cross-checks of every gauge value); Fractional sizes: exact n/64 in arithmetic; mm = 25.4 × in (computed, not transcribed)