F drill bit size
0.257 in
6.53 mm · letter drill · tapping drill for 5/16-18
| Inches | 0.257 in |
| Millimetres | 6.53 mm |
| Nearest metric drill | 6.5 mm |
| System | Letter (A to Z) |
| Taps | 5/16-18 |
What F means
A bit marked F 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 F:
| System | Nearest size | Difference |
|---|---|---|
| number | #1 — 0.228 in | 0.029 in smaller |
| fractional | 1/4" — 0.25 in | 0.007 in smaller |
| metric | 6.5 mm | 0.028 mm smaller |
The closest substitute is 0.007 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 F is for
F is the standard tapping drill for 5/16-18 at 75% thread engagement. That is not a coincidence of the tables: the theoretical figure for 5/16-18 works out at 0.2584 inches, and F at 0.257 is the nearest drill that exists.
What gets drilled at 0.257 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 |
|---|---|---|
| D | 0.246 in | 6.25 mm |
| E | 0.25 in | 6.35 mm |
| 1/4" | 0.25 in | 6.35 mm |
| F | 0.257 in | 6.53 mm |
| G | 0.261 in | 6.63 mm |
| 17/64" | 0.2656 in | 6.75 mm |
| H | 0.266 in | 6.76 mm |
Frequently asked questions
What size is a F drill bit?
0.257 inches, which is 6.53 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 F drill bit in mm?
6.53 mm. A metric index will offer 6.5 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.
What tap does a F drill bit go with?
5/16-18 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 F?
G at 0.261 inches — 0.004 inches larger. Below it is 1/4" at 0.25.
Other sizes
- 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
- I drill bit — 0.272 in, 6.91 mm
- J drill bit — 0.277 in, 7.04 mm
- K drill bit — 0.281 in, 7.14 mm
- 9/32" drill bit — 0.2813 in, 7.14 mm
- L drill bit — 0.29 in, 7.37 mm