1/8" drill bit size
0.125 in
3.18 mm · fractional drill
| Inches | 0.125 in |
| Millimetres | 3.18 mm |
| Nearest metric drill | 3.2 mm |
| System | Fractional (64ths) |
What 1/8" means
1/8" 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 1/8":
| System | Nearest size | Difference |
|---|---|---|
| number | #30 — 0.1285 in | 0.0035 in larger |
| letter | A — 0.234 in | 0.109 in larger |
| metric | 3.2 mm | 0.025 mm larger |
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 gets drilled at 0.125 inches
The busiest part of the number series. Tapping drills for machine screws from #4 to #12 sit in here, along with pilot holes for wood screws and clearance for small pins. If a number drill in a workshop is worn out, it is usually one of these.
Either side of it
| Drill | Inches | Millimetres |
|---|---|---|
| #33 | 0.113 in | 2.87 mm |
| #32 | 0.116 in | 2.95 mm |
| #31 | 0.12 in | 3.05 mm |
| 1/8" | 0.125 in | 3.18 mm |
| #30 | 0.1285 in | 3.26 mm |
| #29 | 0.136 in | 3.45 mm |
| #28 | 0.1405 in | 3.57 mm |
Frequently asked questions
What size is a 1/8" drill bit?
0.125 inches, which is 3.18 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 1/8" drill bit in mm?
3.18 mm. A metric index will offer 3.2 mm as the nearest stocked size, which is 0.025 mm larger — enough to matter when tapping and not enough to matter for a clearance hole.
Is 1/8" 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 1/8"?
#30 at 0.1285 inches — 0.0035 inches larger. Below it is #31 at 0.12.
Other sizes
- #30 drill bit — 0.1285 in, 3.26 mm
- #29 drill bit — 0.136 in, 3.45 mm
- #28 drill bit — 0.1405 in, 3.57 mm
- 9/64" drill bit — 0.1406 in, 3.57 mm
- #27 drill bit — 0.144 in, 3.66 mm
- #26 drill bit — 0.147 in, 3.73 mm
- #25 drill bit — 0.1495 in, 3.8 mm
- #24 drill bit — 0.152 in, 3.86 mm