55/64" drill bit size
0.8594 in
21.83 mm · fractional drill
| Inches | 0.8594 in |
| Millimetres | 21.83 mm |
| Nearest metric drill | 21.8 mm |
| System | Fractional (64ths) |
What 55/64" means
55/64" 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 55/64":
| System | Nearest size | Difference |
|---|---|---|
| number | #1 — 0.228 in | 0.6314 in smaller |
| letter | Z — 0.413 in | 0.4464 in smaller |
| metric | 21.8 mm | 0.028 mm smaller |
The closest substitute is 0.4464 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 0.8594 inches
Half an inch and up is where twist drills stop being the obvious tool. A bit this size in mild steel needs a pilot, a slow speed and something to hold the work; past three-quarters of an inch most people reach for a hole saw or a step drill instead.
Either side of it
| Drill | Inches | Millimetres |
|---|---|---|
| 13/16" | 0.8125 in | 20.64 mm |
| 53/64" | 0.8281 in | 21.03 mm |
| 27/32" | 0.8438 in | 21.43 mm |
| 55/64" | 0.8594 in | 21.83 mm |
| 7/8" | 0.875 in | 22.23 mm |
| 57/64" | 0.8906 in | 22.62 mm |
| 29/32" | 0.9063 in | 23.02 mm |
Frequently asked questions
What size is a 55/64" drill bit?
0.8594 inches, which is 21.83 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 55/64" drill bit in mm?
21.83 mm. A metric index will offer 21.8 mm as the nearest stocked size, which is 0.028 mm smaller — enough to matter when tapping and not enough to matter for a clearance hole.
Is 55/64" a tapping drill size?
Not for any of the unified threads on this site. The next size up, 7/8", is the tapping drill for 1-8.
What is the next size up from 55/64"?
7/8" at 0.875 inches — 0.0156 inches larger. Below it is 27/32" at 0.8438.
Other sizes
- 7/8" drill bit — 0.875 in, 22.23 mm
- 57/64" drill bit — 0.8906 in, 22.62 mm
- 29/32" drill bit — 0.9063 in, 23.02 mm
- 59/64" drill bit — 0.9219 in, 23.42 mm
- 15/16" drill bit — 0.9375 in, 23.81 mm
- 61/64" drill bit — 0.9531 in, 24.21 mm
- 31/32" drill bit — 0.9688 in, 24.61 mm
- 63/64" drill bit — 0.9844 in, 25 mm