X drill bit size
0.397 in
10.08 mm · letter drill
| Inches | 0.397 in |
| Millimetres | 10.08 mm |
| Nearest metric drill | 10.1 mm |
| System | Letter (A to Z) |
What X means
A bit marked X 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 X:
| System | Nearest size | Difference |
|---|---|---|
| number | #1 — 0.228 in | 0.169 in smaller |
| fractional | 25/64" — 0.3906 in | 0.0064 in smaller |
| metric | 10.1 mm | 0.016 mm larger |
The closest substitute is 0.0064 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 0.397 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 |
|---|---|---|
| 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 |
| Z | 0.413 in | 10.49 mm |
Frequently asked questions
What size is a X drill bit?
0.397 inches, which is 10.08 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 X drill bit in mm?
10.08 mm. A metric index will offer 10.1 mm as the nearest stocked size, which is near enough identical.
Is X 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 X?
Y at 0.404 inches — 0.007 inches larger. Below it is 25/64" at 0.3906.
Other sizes
- 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
- 31/64" drill bit — 0.4844 in, 12.3 mm