1/4" drill bit size
0.25 in
6.35 mm · fractional drill
| Inches | 0.25 in |
| Millimetres | 6.35 mm |
| Nearest metric drill | 6.4 mm |
| System | Fractional (64ths) |
What 1/4" means
1/4" 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.
1/4" and E are the same bit
1/4" and E are both exactly 0.25 inches. This is the one place in the three imperial systems where a letter drill and a fractional drill land on the same diameter, so a bit marked either way cuts the same hole. If you have both in a drawer, you have two of the same drill.
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 1/4":
| System | Nearest size | Difference |
|---|---|---|
| number | #1 — 0.228 in | 0.022 in smaller |
| letter | E — 0.25 in | identical |
| metric | 6.4 mm | 0.05 mm larger |
One of those is within three thousandths of an inch, which is inside the tolerance of most hand-held work — substitute it and nobody will know.
What gets drilled at 0.25 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 |
|---|---|---|
| C | 0.242 in | 6.15 mm |
| 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 |
Frequently asked questions
What size is a 1/4" drill bit?
0.25 inches, which is 6.35 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 1/4" drill bit in mm?
6.35 mm. A metric index will offer 6.4 mm as the nearest stocked size, which is 0.05 mm larger — enough to matter when tapping and not enough to matter for a clearance hole.
Is 1/4" a tapping drill size?
Not for any of the unified threads on this site. The next size up, F, is the tapping drill for 5/16-18.
What is the next size up from 1/4"?
F at 0.257 inches — 0.007 inches larger. Below it is D at 0.246.
Other sizes
- F drill bit — 0.257 in, 6.53 mm
- 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