25/64" drill bit size
0.3906 in
9.92 mm · fractional drill · tapping drill for 7/16-20
| Inches | 0.3906 in |
| Millimetres | 9.92 mm |
| Nearest metric drill | 9.9 mm |
| System | Fractional (64ths) |
| Taps | 7/16-20 |
What 25/64" means
25/64" 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 25/64":
| System | Nearest size | Difference |
|---|---|---|
| number | #1 — 0.228 in | 0.1626 in smaller |
| letter | W — 0.386 in | 0.0046 in smaller |
| metric | 9.9 mm | 0.022 mm smaller |
The closest substitute is 0.0046 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 25/64" is for
25/64" is the standard tapping drill for 7/16-20 at 75% thread engagement. That is not a coincidence of the tables: the theoretical figure for 7/16-20 works out at 0.3888 inches, and 25/64" at 0.3906 is the nearest drill that exists.
What gets drilled at 0.3906 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 |
|---|---|---|
| 3/8" | 0.375 in | 9.52 mm |
| V | 0.377 in | 9.58 mm |
| W | 0.386 in | 9.8 mm |
| 25/64" | 0.3906 in | 9.92 mm |
| X | 0.397 in | 10.08 mm |
| Y | 0.404 in | 10.26 mm |
| 13/32" | 0.4063 in | 10.32 mm |
Frequently asked questions
What size is a 25/64" drill bit?
0.3906 inches, which is 9.92 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 25/64" drill bit in mm?
9.92 mm. A metric index will offer 9.9 mm as the nearest stocked size, which is 0.022 mm smaller — enough to matter when tapping and not enough to matter for a clearance hole.
What tap does a 25/64" drill bit go with?
7/16-20 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 25/64"?
X at 0.397 inches — 0.0064 inches larger. Below it is W at 0.386.
Other sizes
- X drill bit — 0.397 in, 10.08 mm
- Y drill bit — 0.404 in, 10.26 mm
- 13/32" drill bit — 0.4063 in, 10.32 mm
- Z drill bit — 0.413 in, 10.49 mm
- 27/64" drill bit — 0.4219 in, 10.72 mm
- 7/16" drill bit — 0.4375 in, 11.11 mm
- 29/64" drill bit — 0.4531 in, 11.51 mm
- 15/32" drill bit — 0.4688 in, 11.91 mm