#38 drill bit size
0.1015 in
2.58 mm · number drill · tapping drill for 5-40
| Inches | 0.1015 in |
| Millimetres | 2.58 mm |
| Nearest metric drill | 2.6 mm |
| System | Number (#1 to #60) |
| Taps | 5-40 |
What #38 means
A bit marked #38 is a number drill, and the number is an index rather than a measurement — the higher the number, the smaller the bit. #1 is 0.228 inches and #60 is 0.04. That is the opposite of the fractional sizes stamped on a home set, which is why a number drill and a fraction of similar appearance can be very different holes.
The same hole in the other systems
Three drill systems overlap across this range and a workshop rarely has all three. The nearest bit of each other kind to #38:
| System | Nearest size | Difference |
|---|---|---|
| letter | A — 0.234 in | 0.1325 in larger |
| fractional | 3/32" — 0.0938 in | 0.0078 in smaller |
| metric | 2.6 mm | 0.022 mm larger |
The closest substitute is 0.0078 inches away. For a clearance hole that is nothing; for a tapping hole it moves the thread engagement by several per cent, which is the difference between an easy tap and a broken one.
What #38 is for
#38 is the standard tapping drill for 5-40 at 75% thread engagement. That is not a coincidence of the tables: the theoretical figure for 5-40 works out at 0.1006 inches, and #38 at 0.1015 is the nearest drill that exists.
What gets drilled at 0.1015 inches
The busiest part of the number series. Tapping drills for machine screws from #4 to #12 sit in here, along with pilot holes for wood screws and clearance for small pins. If a number drill in a workshop is worn out, it is usually one of these.
Either side of it
| Drill | Inches | Millimetres |
|---|---|---|
| #41 | 0.096 in | 2.44 mm |
| #40 | 0.098 in | 2.49 mm |
| #39 | 0.0995 in | 2.53 mm |
| #38 | 0.1015 in | 2.58 mm |
| #37 | 0.104 in | 2.64 mm |
| #36 | 0.1065 in | 2.71 mm |
| 7/64" | 0.1094 in | 2.78 mm |
Frequently asked questions
What size is a #38 drill bit?
0.1015 inches, which is 2.58 mm. Number drills run from #1 at 0.228 inches down to #60 at 0.04, getting smaller as the number rises — the opposite of what most people expect the first time.
What is a #38 drill bit in mm?
2.58 mm. A metric index will offer 2.6 mm as the nearest stocked size, which is 0.022 mm larger — enough to matter when tapping and not enough to matter for a clearance hole.
What tap does a #38 drill bit go with?
5-40 at the standard 75% thread. That thread comes out at a theoretical diameter this drill is the nearest stocked size to.
What is the next size up from #38?
#37 at 0.104 inches — 0.0025 inches larger. Below it is #39 at 0.0995.
Other sizes
- #37 drill bit — 0.104 in, 2.64 mm
- #36 drill bit — 0.1065 in, 2.71 mm
- 7/64" drill bit — 0.1094 in, 2.78 mm
- #35 drill bit — 0.11 in, 2.79 mm
- #34 drill bit — 0.111 in, 2.82 mm
- #33 drill bit — 0.113 in, 2.87 mm
- #32 drill bit — 0.116 in, 2.95 mm
- #31 drill bit — 0.12 in, 3.05 mm