#21 drill bit size
0.159 in
4.04 mm · number drill · tapping drill for 10-32
| Inches | 0.159 in |
| Millimetres | 4.04 mm |
| Nearest metric drill | 4 mm |
| System | Number (#1 to #60) |
| Taps | 10-32 |
What #21 means
A bit marked #21 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 #21:
| System | Nearest size | Difference |
|---|---|---|
| letter | A — 0.234 in | 0.075 in larger |
| fractional | 5/32" — 0.1563 in | 0.0028 in smaller |
| metric | 4 mm | 0.039 mm smaller |
One of those is within three thousandths of an inch, which is inside the tolerance of most hand-held work — substitute it and nobody will know.
What #21 is for
#21 is the standard tapping drill for 10-32 at 75% thread engagement. That is not a coincidence of the tables: the theoretical figure for 10-32 works out at 0.1596 inches, and #21 at 0.159 is the nearest drill that exists.
What gets drilled at 0.159 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 |
|---|---|---|
| #23 | 0.154 in | 3.91 mm |
| 5/32" | 0.1563 in | 3.97 mm |
| #22 | 0.157 in | 3.99 mm |
| #21 | 0.159 in | 4.04 mm |
| #20 | 0.161 in | 4.09 mm |
| #19 | 0.166 in | 4.22 mm |
| #18 | 0.1695 in | 4.31 mm |
Frequently asked questions
What size is a #21 drill bit?
0.159 inches, which is 4.04 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 #21 drill bit in mm?
4.04 mm. A metric index will offer 4 mm as the nearest stocked size, which is 0.039 mm smaller — enough to matter when tapping and not enough to matter for a clearance hole.
What tap does a #21 drill bit go with?
10-32 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 #21?
#20 at 0.161 inches — 0.002 inches larger. Below it is #22 at 0.157.
Other sizes
- #20 drill bit — 0.161 in, 4.09 mm
- #19 drill bit — 0.166 in, 4.22 mm
- #18 drill bit — 0.1695 in, 4.31 mm
- 11/64" drill bit — 0.1719 in, 4.37 mm
- #17 drill bit — 0.173 in, 4.39 mm
- #16 drill bit — 0.177 in, 4.5 mm
- #15 drill bit — 0.18 in, 4.57 mm
- #14 drill bit — 0.182 in, 4.62 mm