#40 drill bit size
0.098 in
2.49 mm · number drill
| Inches | 0.098 in |
| Millimetres | 2.49 mm |
| Nearest metric drill | 2.5 mm |
| System | Number (#1 to #60) |
What #40 means
A bit marked #40 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 #40:
| System | Nearest size | Difference |
|---|---|---|
| letter | A — 0.234 in | 0.136 in larger |
| fractional | 3/32" — 0.0938 in | 0.0043 in smaller |
| metric | 2.5 mm | 0.011 mm larger |
The closest substitute is 0.0043 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 gets drilled at 0.098 inches
This is circuit-board and small-pin range — component leads, header pins, the holes a hobbyist drills in perfboard. It is also where the smallest machine-screw taps live, so a drill here is as likely to be preparing a thread as clearing a wire.
Either side of it
| Drill | Inches | Millimetres |
|---|---|---|
| #42 | 0.0935 in | 2.37 mm |
| 3/32" | 0.0938 in | 2.38 mm |
| #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 |
Frequently asked questions
What size is a #40 drill bit?
0.098 inches, which is 2.49 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 #40 drill bit in mm?
2.49 mm. A metric index will offer 2.5 mm as the nearest stocked size, which is near enough identical.
Is #40 a tapping drill size?
Not for any of the unified threads on this site. It sits between the sizes the standard taps call for, so it is a clearance or pilot drill rather than a tapping one.
What is the next size up from #40?
#39 at 0.0995 inches — 0.0015 inches larger. Below it is #41 at 0.096.
Other sizes
- #39 drill bit — 0.0995 in, 2.53 mm
- #38 drill bit — 0.1015 in, 2.58 mm
- #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