21/64" drill bit size
0.3281 in
8.33 mm · fractional drill
| Inches | 0.3281 in |
| Millimetres | 8.33 mm |
| Nearest metric drill | 8.3 mm |
| System | Fractional (64ths) |
What 21/64" means
21/64" is a fractional drill, stepping in 64ths of an inch. Fractional sizes are the coarsest of the three systems, which is why a fractional set alone cannot hit most tapping sizes and why number and letter drills exist at all.
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 21/64":
| System | Nearest size | Difference |
|---|---|---|
| number | #1 — 0.228 in | 0.1001 in smaller |
| letter | Q — 0.332 in | 0.0039 in larger |
| metric | 8.3 mm | 0.034 mm smaller |
The closest substitute is 0.0039 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 0.3281 inches
Clearance holes for bolts up to about half an inch, and tapping drills for the larger unified threads. Above a quarter inch a hand drill starts to want two hands and a slower speed, and the bit wants a pilot hole.
Either side of it
| Drill | Inches | Millimetres |
|---|---|---|
| 5/16" | 0.3125 in | 7.94 mm |
| O | 0.316 in | 8.03 mm |
| P | 0.323 in | 8.2 mm |
| 21/64" | 0.3281 in | 8.33 mm |
| Q | 0.332 in | 8.43 mm |
| R | 0.339 in | 8.61 mm |
| 11/32" | 0.3438 in | 8.73 mm |
Frequently asked questions
What size is a 21/64" drill bit?
0.3281 inches, which is 8.33 mm. Fractional drills step in 64ths of an inch, which is the coarsest of the three systems and the one most home sets contain.
What is a 21/64" drill bit in mm?
8.33 mm. A metric index will offer 8.3 mm as the nearest stocked size, which is 0.034 mm smaller — enough to matter when tapping and not enough to matter for a clearance hole.
Is 21/64" a tapping drill size?
Not for any of the unified threads on this site. The next size up, Q, is the tapping drill for 3/8-24.
What is the next size up from 21/64"?
Q at 0.332 inches — 0.0039 inches larger. Below it is P at 0.323.
Other sizes
- Q drill bit — 0.332 in, 8.43 mm
- R drill bit — 0.339 in, 8.61 mm
- 11/32" drill bit — 0.3438 in, 8.73 mm
- S drill bit — 0.348 in, 8.84 mm
- T drill bit — 0.358 in, 9.09 mm
- 23/64" drill bit — 0.3594 in, 9.13 mm
- U drill bit — 0.368 in, 9.35 mm
- 3/8" drill bit — 0.375 in, 9.52 mm