#4 drill bit size
0.209 in
5.31 mm · number drill
| Inches | 0.209 in |
| Millimetres | 5.31 mm |
| Nearest metric drill | 5.3 mm |
| System | Number (#1 to #60) |
What #4 means
A bit marked #4 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 #4:
| System | Nearest size | Difference |
|---|---|---|
| letter | A — 0.234 in | 0.025 in larger |
| fractional | 13/64" — 0.2031 in | 0.0059 in smaller |
| metric | 5.3 mm | 0.009 mm smaller |
The closest substitute is 0.0059 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 0.209 inches
Where the quarter-inch and five-sixteenths taps are drilled, and where clearance holes for small bolts begin. The three drill systems overlap most densely across this band, which is why a bit here is the one most often picked up and misidentified.
Either side of it
| Drill | Inches | Millimetres |
|---|---|---|
| 13/64" | 0.2031 in | 5.16 mm |
| #6 | 0.204 in | 5.18 mm |
| #5 | 0.2055 in | 5.22 mm |
| #4 | 0.209 in | 5.31 mm |
| #3 | 0.213 in | 5.41 mm |
| 7/32" | 0.2188 in | 5.56 mm |
| #2 | 0.221 in | 5.61 mm |
Frequently asked questions
What size is a #4 drill bit?
0.209 inches, which is 5.31 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 #4 drill bit in mm?
5.31 mm. A metric index will offer 5.3 mm as the nearest stocked size, which is near enough identical.
Is #4 a tapping drill size?
Not for any of the unified threads on this site. The next size up, #3, is the tapping drill for 1/4-28.
What is the next size up from #4?
#3 at 0.213 inches — 0.004 inches larger. Below it is #5 at 0.2055.
Other sizes
- #3 drill bit — 0.213 in, 5.41 mm
- 7/32" drill bit — 0.2188 in, 5.56 mm
- #2 drill bit — 0.221 in, 5.61 mm
- #1 drill bit — 0.228 in, 5.79 mm
- A drill bit — 0.234 in, 5.94 mm
- 15/64" drill bit — 0.2344 in, 5.95 mm
- B drill bit — 0.238 in, 6.05 mm
- C drill bit — 0.242 in, 6.15 mm