B drill bit size
0.238 in
6.05 mm · letter drill
| Inches | 0.238 in |
| Millimetres | 6.05 mm |
| Nearest metric drill | 6 mm |
| System | Letter (A to Z) |
What B means
A bit marked B is a letter drill. Letters run A to Z and get larger as they advance, covering 0.234 to 0.413 inches — the gap between the largest number drill and the point where fractional sizes become fine enough to be useful.
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 B:
| System | Nearest size | Difference |
|---|---|---|
| number | #1 — 0.228 in | 0.01 in smaller |
| fractional | 15/64" — 0.2344 in | 0.0036 in smaller |
| metric | 6 mm | 0.045 mm smaller |
The closest substitute is 0.0036 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 0.238 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 |
|---|---|---|
| #1 | 0.228 in | 5.79 mm |
| A | 0.234 in | 5.94 mm |
| 15/64" | 0.2344 in | 5.95 mm |
| B | 0.238 in | 6.05 mm |
| C | 0.242 in | 6.15 mm |
| D | 0.246 in | 6.25 mm |
| E | 0.25 in | 6.35 mm |
Frequently asked questions
What size is a B drill bit?
0.238 inches, which is 6.05 mm. Letter drills run A to Z and get larger as the letter advances, filling the gap between the largest number drill and the fractional sizes.
What is a B drill bit in mm?
6.05 mm. A metric index will offer 6 mm as the nearest stocked size, which is 0.045 mm smaller — enough to matter when tapping and not enough to matter for a clearance hole.
Is B 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 B?
C at 0.242 inches — 0.004 inches larger. Below it is 15/64" at 0.2344.
Other sizes
- C drill bit — 0.242 in, 6.15 mm
- D drill bit — 0.246 in, 6.25 mm
- E drill bit — 0.25 in, 6.35 mm
- 1/4" drill bit — 0.25 in, 6.35 mm
- F drill bit — 0.257 in, 6.53 mm
- G drill bit — 0.261 in, 6.63 mm
- 17/64" drill bit — 0.2656 in, 6.75 mm
- H drill bit — 0.266 in, 6.76 mm