5/16" drill bit size
0.3125 in
7.94 mm · fractional drill · tapping drill for 3/8-16
| Inches | 0.3125 in |
| Millimetres | 7.94 mm |
| Nearest metric drill | 7.9 mm |
| System | Fractional (64ths) |
| Taps | 3/8-16 |
What 5/16" means
5/16" 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 5/16":
| System | Nearest size | Difference |
|---|---|---|
| number | #1 — 0.228 in | 0.0845 in smaller |
| letter | O — 0.316 in | 0.0035 in larger |
| metric | 7.9 mm | 0.037 mm smaller |
The closest substitute is 0.0035 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 5/16" is for
5/16" is the standard tapping drill for 3/8-16 at 75% thread engagement. That is not a coincidence of the tables: the theoretical figure for 3/8-16 works out at 0.3141 inches, and 5/16" at 0.3125 is the nearest drill that exists.
What gets drilled at 0.3125 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 |
|---|---|---|
| M | 0.295 in | 7.49 mm |
| 19/64" | 0.2969 in | 7.54 mm |
| N | 0.302 in | 7.67 mm |
| 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 |
Frequently asked questions
What size is a 5/16" drill bit?
0.3125 inches, which is 7.94 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 5/16" drill bit in mm?
7.94 mm. A metric index will offer 7.9 mm as the nearest stocked size, which is 0.037 mm smaller — enough to matter when tapping and not enough to matter for a clearance hole.
What tap does a 5/16" drill bit go with?
3/8-16 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 5/16"?
O at 0.316 inches — 0.0035 inches larger. Below it is N at 0.302.
Other sizes
- O drill bit — 0.316 in, 8.03 mm
- P drill bit — 0.323 in, 8.2 mm
- 21/64" drill bit — 0.3281 in, 8.33 mm
- 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