Number & letter drill bit sizes (free printable PDF)
The complete number and letter drill bit reference: every wire-gauge bit #1 to #80 and every letter bit A to Z, with decimal-inch and millimetre equivalents. Number bits run largest (#1) to smallest (#80) — the thing that trips people up. Download the free printable PDF or print it from the page.
How number & letter bits work
Number (wire-gauge) bits cover the small sizes, #1 down to #80 — and the number runs backwards: #1 is the largest (0.228") and #80 the tiniest (0.0135"). Letter bits pick up just above them, A to Z, running the normal way (A smallest, Z largest). Both fill the fine gaps between fractional sizes, which is why they show up most in precision and tapping work.
Number drill bit sizes (#1–#80)
| Number | Decimal inch | 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 |
| #11 | 0.191" | 4.85 |
| #12 | 0.189" | 4.8 |
| #13 | 0.185" | 4.7 |
| #14 | 0.182" | 4.62 |
| #15 | 0.18" | 4.57 |
| #16 | 0.177" | 4.5 |
| #17 | 0.173" | 4.39 |
| #18 | 0.1695" | 4.31 |
| #19 | 0.166" | 4.22 |
| #20 | 0.161" | 4.09 |
| #21 | 0.159" | 4.04 |
| #22 | 0.157" | 3.99 |
| #23 | 0.154" | 3.91 |
| #24 | 0.152" | 3.86 |
| #25 | 0.1495" | 3.8 |
| #26 | 0.147" | 3.73 |
| #27 | 0.144" | 3.66 |
| #28 | 0.1405" | 3.57 |
| #29 | 0.136" | 3.45 |
| #30 | 0.1285" | 3.26 |
| #31 | 0.12" | 3.05 |
| #32 | 0.116" | 2.95 |
| #33 | 0.113" | 2.87 |
| #34 | 0.111" | 2.82 |
| #35 | 0.11" | 2.79 |
| #36 | 0.1065" | 2.71 |
| #37 | 0.104" | 2.64 |
| #38 | 0.1015" | 2.58 |
| #39 | 0.0995" | 2.53 |
| #40 | 0.098" | 2.49 |
| #41 | 0.096" | 2.44 |
| #42 | 0.0935" | 2.37 |
| #43 | 0.089" | 2.26 |
| #44 | 0.086" | 2.18 |
| #45 | 0.082" | 2.08 |
| #46 | 0.081" | 2.06 |
| #47 | 0.0785" | 1.99 |
| #48 | 0.076" | 1.93 |
| #49 | 0.073" | 1.85 |
| #50 | 0.07" | 1.78 |
| #51 | 0.067" | 1.7 |
| #52 | 0.0635" | 1.61 |
| #53 | 0.0595" | 1.51 |
| #54 | 0.055" | 1.4 |
| #55 | 0.052" | 1.32 |
| #56 | 0.0465" | 1.18 |
| #57 | 0.043" | 1.09 |
| #58 | 0.042" | 1.07 |
| #59 | 0.041" | 1.04 |
| #60 | 0.04" | 1.02 |
| #61 | 0.039" | 0.99 |
| #62 | 0.038" | 0.97 |
| #63 | 0.037" | 0.94 |
| #64 | 0.036" | 0.91 |
| #65 | 0.035" | 0.89 |
| #66 | 0.033" | 0.84 |
| #67 | 0.032" | 0.81 |
| #68 | 0.031" | 0.79 |
| #69 | 0.0292" | 0.74 |
| #70 | 0.028" | 0.71 |
| #71 | 0.026" | 0.66 |
| #72 | 0.025" | 0.64 |
| #73 | 0.024" | 0.61 |
| #74 | 0.0225" | 0.57 |
| #75 | 0.021" | 0.53 |
| #76 | 0.02" | 0.51 |
| #77 | 0.018" | 0.46 |
| #78 | 0.016" | 0.41 |
| #79 | 0.0145" | 0.37 |
| #80 | 0.0135" | 0.34 |
Number bits run backwards: #1 is the largest and #80 the smallest, because the number is a wire gauge, not a measurement. Decimal and mm are the fixed standard equivalents.
whatdrillbit.com
Letter drill bit sizes (A–Z)
| Letter | Decimal inch | 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 |
Letter bits run the intuitive way — A is smallest, Z largest — and sit just above the number range, filling the gaps between fractional sizes. Note letter E = 1/4" exactly.
whatdrillbit.com
Common questions
What size is a #7 drill bit?
A #7 drill bit is 0.201" in decimal inches, which is 5.11 mm. It falls between the 13/64" and 7/32" fractional sizes — a common tapping-drill size.
What is a letter Q drill bit in mm?
A letter Q drill bit is 0.332" (8.43 mm). Letter bits fill the gaps between fractional sizes in the mid range; Q sits just above 5/16" (0.3125").
Why do drill bit numbers get bigger as the bit gets smaller?
Number bits use a wire-gauge system, not a direct measurement — the number is a gauge, so it runs backwards. #1 is the largest at 0.228" (5.79 mm) and #80 is the smallest at 0.0135" (0.34 mm). Letter bits, by contrast, run the intuitive way: A (5.94 mm) is smallest and Z (10.49 mm) is largest.
What are number and letter drill bits used for?
They fill the fine gaps between fractional sizes, so they're used where hole size matters precisely — most often for tapping threads (drilling the right hole for a given screw or bolt thread), dowel and pin fits, and other machining and metalwork. Number bits cover the smaller range; letter bits pick up just above them.
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Where these numbers come from
Number and letter drill decimals are the fixed ANSI/ASME wire-gauge and letter standards, agreed across every drill reference. Millimetres are computed from the decimal inch (× 25.4). Nothing here is estimated.
Number and letter drill decimals are fixed industry-standard values. Bit diameters are nominal; for precision or tapping work, confirm against your tap-drill chart and measure the bit.