Numbered & Letter Drill Bit Chart
Every size in the American gauge series on one page — all 80 number drills (#1 to #80) and all 26 letter drills (A to Z), in the order they actually measure, with the decimal, the millimetre equivalent and the tap each one drills where it has a standard job. No other page here lists the whole series; this is the index to it.
Number drill sizes — #1 to #80
Sorted by diameter, largest first. Type in the box to jump straight to a size:
| Drill | Inches | Millimetres | Common tap use |
|---|---|---|---|
| #1 | 0.2280 | 5.79 | — |
| #2 | 0.2210 | 5.61 | — |
| #3 | 0.2130 | 5.41 | 1/4-28 UNF |
| #4 | 0.2090 | 5.31 | — |
| #5 | 0.2055 | 5.22 | — |
| #6 | 0.2040 | 5.18 | — |
| #7 | 0.2010 | 5.11 | 1/4-20 UNC |
| #8 | 0.1990 | 5.05 | — |
| #9 | 0.1960 | 4.98 | — |
| #10 | 0.1935 | 4.91 | — |
| #11 | 0.1910 | 4.85 | — |
| #12 | 0.1890 | 4.80 | — |
| #13 | 0.1850 | 4.70 | — |
| #14 | 0.1820 | 4.62 | — |
| #15 | 0.1800 | 4.57 | — |
| #16 | 0.1770 | 4.50 | 12-24 UNC |
| #17 | 0.1730 | 4.39 | — |
| #18 | 0.1695 | 4.31 | — |
| #19 | 0.1660 | 4.22 | — |
| #20 | 0.1610 | 4.09 | — |
| #21 | 0.1590 | 4.04 | 10-32 UNF |
| #22 | 0.1570 | 3.99 | — |
| #23 | 0.1540 | 3.91 | — |
| #24 | 0.1520 | 3.86 | — |
| #25 | 0.1495 | 3.80 | 10-24 UNC |
| #26 | 0.1470 | 3.73 | — |
| #27 | 0.1440 | 3.66 | — |
| #28 | 0.1405 | 3.57 | — |
| #29 | 0.1360 | 3.45 | 8-32 UNC |
| #30 | 0.1285 | 3.26 | — |
| #31 | 0.1200 | 3.05 | — |
| #32 | 0.1160 | 2.95 | 6-48 UNF |
| #33 | 0.1130 | 2.87 | — |
| #34 | 0.1110 | 2.82 | — |
| #35 | 0.1100 | 2.79 | — |
| #36 | 0.1065 | 2.71 | 6-32 UNC |
| #37 | 0.1040 | 2.64 | — |
| #38 | 0.1015 | 2.58 | — |
| #39 | 0.0995 | 2.53 | — |
| #40 | 0.0980 | 2.49 | — |
| #41 | 0.0960 | 2.44 | — |
| #42 | 0.0935 | 2.37 | — |
| #43 | 0.0890 | 2.26 | 4-40 UNC |
| #44 | 0.0860 | 2.18 | — |
| #45 | 0.0820 | 2.08 | — |
| #46 | 0.0810 | 2.06 | — |
| #47 | 0.0785 | 1.99 | — |
| #48 | 0.0760 | 1.93 | — |
| #49 | 0.0730 | 1.85 | — |
| #50 | 0.0700 | 1.78 | 2-56 UNC |
| #51 | 0.0670 | 1.70 | — |
| #52 | 0.0635 | 1.61 | — |
| #53 | 0.0595 | 1.51 | — |
| #54 | 0.0550 | 1.40 | — |
| #55 | 0.0520 | 1.32 | — |
| #56 | 0.0465 | 1.18 | — |
| #57 | 0.0430 | 1.09 | — |
| #58 | 0.0420 | 1.07 | — |
| #59 | 0.0410 | 1.04 | — |
| #60 | 0.0400 | 1.02 | — |
| #61 | 0.0390 | 0.99 | — |
| #62 | 0.0380 | 0.97 | — |
| #63 | 0.0370 | 0.94 | — |
| #64 | 0.0360 | 0.91 | — |
| #65 | 0.0350 | 0.89 | — |
| #66 | 0.0330 | 0.84 | — |
| #67 | 0.0320 | 0.81 | — |
| #68 | 0.0310 | 0.79 | — |
| #69 | 0.0292 | 0.74 | — |
| #70 | 0.0280 | 0.71 | — |
| #71 | 0.0260 | 0.66 | — |
| #72 | 0.0250 | 0.64 | — |
| #73 | 0.0240 | 0.61 | — |
| #74 | 0.0225 | 0.57 | — |
| #75 | 0.0210 | 0.53 | — |
| #76 | 0.0200 | 0.51 | — |
| #77 | 0.0180 | 0.46 | — |
| #78 | 0.0160 | 0.41 | — |
| #79 | 0.0145 | 0.37 | — |
| #80 | 0.0135 | 0.34 | — |
Letter drill sizes — A to Z
Letter bits fill the gaps above the number series. F, I, Q and U are the ones used as tap drills:
| Drill | Inches | Millimetres | Common tap use |
|---|---|---|---|
| A | 0.2340 | 5.94 | — |
| B | 0.2380 | 6.05 | — |
| C | 0.2420 | 6.15 | — |
| D | 0.2460 | 6.25 | — |
| E | 0.2500 | 6.35 | — |
| F | 0.2570 | 6.53 | 5/16-18 UNC |
| G | 0.2610 | 6.63 | — |
| H | 0.2660 | 6.76 | — |
| I | 0.2720 | 6.91 | 5/16-24 UNF |
| J | 0.2770 | 7.04 | — |
| K | 0.2810 | 7.14 | — |
| L | 0.2900 | 7.37 | — |
| M | 0.2950 | 7.49 | — |
| N | 0.3020 | 7.67 | — |
| O | 0.3160 | 8.03 | — |
| P | 0.3230 | 8.20 | — |
| Q | 0.3320 | 8.43 | 3/8-24 UNF |
| R | 0.3390 | 8.61 | — |
| S | 0.3480 | 8.84 | — |
| T | 0.3580 | 9.09 | — |
| U | 0.3680 | 9.35 | 7/16-14 UNC |
| V | 0.3770 | 9.58 | — |
| W | 0.3860 | 9.80 | — |
| X | 0.3970 | 10.08 | — |
| Y | 0.4040 | 10.26 | — |
| Z | 0.4130 | 10.49 | — |
How the gauge numbering works
The number series runs from #1 at 0.2280 in down to #80 at 0.0135 in — so a higher number is a smaller hole. That is the single fact that trips people up: #7 is larger than #29, and #43 is smaller still. The numbers are a fixed catalogue order, not a measurement, which is why a chart is needed at all.
#1 = 0.2280 in = 5.79 mm (largest number bit) #80 = 0.0135 in = 0.34 mm (smallest number bit) letter A = 0.2340 in = 5.94 mm (smallest letter bit) letter Z = 0.4130 in = 10.49 mm (largest letter bit)
Letter bits start where the numbers stop: the smallest letter (A, 0.2340 in) is still larger than the largest number (#1, 0.2280 in). The two series together cover 0.0135 in to 0.4130 in — the range where fractional sizes alone leave gaps.
Number, letter, fractional or metric — which do you reach for?
Fractional bits (1/8, 5/16, 1/2) are named by the actual inch measurement and cover the common round sizes. Number and letter bits exist because a fraction cannot express every size a machinist needs — there is no simple fraction for 0.1360 in, so it is called #29. Metric bits are named in millimetres and follow the same idea in a decimal system.
In practice the choice is often fixed by the tap: a 1/4-20 thread wants a #7, a 5/16-18 wants an F, an M6 wants 5.0 mm. Where a job does not dictate the size, pick the nearest drill to the hole you actually need — the mm column above lets you compare across all four naming systems at a glance.
Frequently asked questions
What is a number drill bit?
A number drill bit is a size in the American gauge series, running from #1 (largest, 0.2280 in) to #80 (smallest, 0.0135 in). The number does not describe the diameter — it is simply a catalogue position, so you always need the chart to convert a number to a measurement.
Why does a bigger number mean a smaller drill?
The gauge series was numbered smallest-to-largest in the opposite direction to what most people expect: #80 is the thinnest bit and #1 the thickest. It is a fixed ordering, not a measurement, which is exactly why number bits are the ones people look up most often.
What is a letter drill bit?
Letter bits run A (smallest, 0.2340 in) to Z (largest, 0.4130 in) and fill the gaps between the larger fractional sizes. They sit above the number series — the smallest letter, A, is still bigger than the largest number, #1. F, I, Q and U are all standard tap drills.
What is the difference between number, letter, fractional and metric drills?
Fractional bits (1/8, 1/4, 5/16…) are named by their actual inch fraction. Number and letter bits are named by a gauge code because they fill sizes a fraction cannot express. Metric bits are named in millimetres. All four describe the same physical thing — a round hole — and the chart above lets you find the match across systems.
What is the most common drill bit size?
For tapping, the #7 (0.2010 in) is the most looked-up number bit, because it is the standard tap drill for the near-universal 1/4-20 thread. In the letter series, F is the most common, as the 5/16-18 tap drill.
How do I convert a number drill size to millimetres?
Multiply the inch diameter by 25.4. For example, #29 is 0.1360 in, so 0.1360 × 25.4 = 3.45 mm. The millimetre column in the chart above is already worked out for every size.