Number Drill Bit Sizes (#1 to #80)
Number drill sizes run from #80 (0.0135", smallest) to #1 (0.228", largest) — counting down as the physical size goes up. A #7 drill, at 0.201", is the standard tap drill for a 1/4-20 bolt.
Why the Numbers Count Down
Number drills fill the range below and between fractional sizes with much finer steps — 80 of them between roughly 0.0135" and 0.228" — but the numbering runs opposite to what you'd expect: the bigger the number, the smaller the drill.
That inverse relationship comes from wire gauge convention, where a higher gauge number means a thinner wire. Number drills inherited the same logic, so #80 (the "biggest" number) is the smallest physical bit, while #1 (the "smallest" number) is the largest.
Number sizes are most often specified on tap drill charts, where a thread's ideal pilot hole rarely lines up with a clean fraction — see the tap drill size chart in the machinist tools section, and letter drill sizes for the range immediately above where numbers leave off.
Converting a Number Size to Millimetres
Once you have a number drill's decimal inch value, converting it to millimetres uses the same exact constant as everywhere else on this site.
MM equals the number drill's decimal inch value multiplied by 25.4.
- INThe number drill's standardized decimal inch size, looked up from the table below.
- MMThe millimetre equivalent: IN × 25.4.
Worked calculation — #7
- #7 = 0.201"Look up the standardized size
- 0.201 × 25.4 = 5.1054Convert to millimetres
#7 = 0.201" = 5.105 mm
All 80 Number Drill Sizes
| Size | Decimal | MM | Size | Decimal | MM |
|---|---|---|---|---|---|
| #80 | 0.0135" | 0.343 | #40 | 0.098" | 2.489 |
| #79 | 0.0145" | 0.368 | #39 | 0.0995" | 2.527 |
| #78 | 0.016" | 0.406 | #38 | 0.1015" | 2.578 |
| #77 | 0.018" | 0.457 | #37 | 0.104" | 2.642 |
| #76 | 0.02" | 0.508 | #36 | 0.1065" | 2.705 |
| #75 | 0.021" | 0.533 | #35 | 0.11" | 2.794 |
| #74 | 0.0225" | 0.571 | #34 | 0.111" | 2.819 |
| #73 | 0.024" | 0.61 | #33 | 0.113" | 2.87 |
| #72 | 0.025" | 0.635 | #32 | 0.116" | 2.946 |
| #71 | 0.026" | 0.66 | #31 | 0.12" | 3.048 |
| #70 | 0.028" | 0.711 | #30 | 0.1285" | 3.264 |
| #69 | 0.0292" | 0.742 | #29 | 0.136" | 3.454 |
| #68 | 0.031" | 0.787 | #28 | 0.1405" | 3.569 |
| #67 | 0.032" | 0.813 | #27 | 0.144" | 3.658 |
| #66 | 0.033" | 0.838 | #26 | 0.147" | 3.734 |
| #65 | 0.035" | 0.889 | #25 | 0.1495" | 3.797 |
| #64 | 0.036" | 0.914 | #24 | 0.152" | 3.861 |
| #63 | 0.037" | 0.94 | #23 | 0.154" | 3.912 |
| #62 | 0.038" | 0.965 | #22 | 0.157" | 3.988 |
| #61 | 0.039" | 0.991 | #21 | 0.159" | 4.039 |
| #60 | 0.04" | 1.016 | #20 | 0.161" | 4.089 |
| #59 | 0.041" | 1.041 | #19 | 0.166" | 4.216 |
| #58 | 0.042" | 1.067 | #18 | 0.1695" | 4.305 |
| #57 | 0.043" | 1.092 | #17 | 0.173" | 4.394 |
| #56 | 0.0465" | 1.181 | #16 | 0.177" | 4.496 |
| #55 | 0.052" | 1.321 | #15 | 0.18" | 4.572 |
| #54 | 0.055" | 1.397 | #14 | 0.182" | 4.623 |
| #53 | 0.0595" | 1.511 | #13 | 0.185" | 4.699 |
| #52 | 0.0635" | 1.613 | #12 | 0.189" | 4.801 |
| #51 | 0.067" | 1.702 | #11 | 0.191" | 4.851 |
| #50 | 0.07" | 1.778 | #10 | 0.1935" | 4.915 |
| #49 | 0.073" | 1.854 | #9 | 0.196" | 4.978 |
| #48 | 0.076" | 1.93 | #8 | 0.199" | 5.055 |
| #47 | 0.0785" | 1.994 | #7 | 0.201" | 5.105 |
| #46 | 0.081" | 2.057 | #6 | 0.204" | 5.182 |
| #45 | 0.082" | 2.083 | #5 | 0.2055" | 5.22 |
| #44 | 0.086" | 2.184 | #4 | 0.209" | 5.309 |
| #43 | 0.089" | 2.261 | #3 | 0.213" | 5.41 |
| #42 | 0.0935" | 2.375 | #2 | 0.221" | 5.613 |
| #41 | 0.096" | 2.438 | #1 | 0.228" | 5.791 |
Frequently Asked Questions
What is the smallest number drill size?
#80 is the smallest number drill, at 0.0135" (0.343 mm).
What is the largest number drill size?
#1 is the largest number drill, at 0.228" (5.791 mm).
Why do number drills get smaller as the number gets bigger?
The numbering follows a historical wire gauge convention, where a higher gauge number means a thinner wire — and by extension, a smaller drill. It's the same inverse relationship found in sheet metal and wire gauges generally.
What is a #7 drill used for?
A #7 drill (0.201") is the standard tap drill size for a 1/4-20 UNC thread — one of the most common bolt sizes in general hardware.
What is the decimal size of a #30 drill?
A #30 drill measures 0.1285" (3.264 mm).
Drill Size References
Every other chart in this cluster, one click away.