#27 drill bit size
0.144 in
3.66 mm · number drill
| Inches | 0.144 in |
| Millimetres | 3.66 mm |
| Nearest metric drill | 3.7 mm |
| System | Number (#1 to #60) |
What #27 means
A bit marked #27 is a number drill, and the number is an index rather than a measurement — the higher the number, the smaller the bit. #1 is 0.228 inches and #60 is 0.04. That is the opposite of the fractional sizes stamped on a home set, which is why a number drill and a fraction of similar appearance can be very different holes.
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 #27:
| System | Nearest size | Difference |
|---|---|---|
| letter | A — 0.234 in | 0.09 in larger |
| fractional | 9/64" — 0.1406 in | 0.0034 in smaller |
| metric | 3.7 mm | 0.042 mm larger |
The closest substitute is 0.0034 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 0.144 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 |
|---|---|---|
| #29 | 0.136 in | 3.45 mm |
| #28 | 0.1405 in | 3.57 mm |
| 9/64" | 0.1406 in | 3.57 mm |
| #27 | 0.144 in | 3.66 mm |
| #26 | 0.147 in | 3.73 mm |
| #25 | 0.1495 in | 3.8 mm |
| #24 | 0.152 in | 3.86 mm |
Frequently asked questions
What size is a #27 drill bit?
0.144 inches, which is 3.66 mm. Number drills run from #1 at 0.228 inches down to #60 at 0.04, getting smaller as the number rises — the opposite of what most people expect the first time.
What is a #27 drill bit in mm?
3.66 mm. A metric index will offer 3.7 mm as the nearest stocked size, which is 0.042 mm larger — enough to matter when tapping and not enough to matter for a clearance hole.
Is #27 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 #27?
#26 at 0.147 inches — 0.003 inches larger. Below it is 9/64" at 0.1406.
Other sizes
- #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
- #23 drill bit — 0.154 in, 3.91 mm
- 5/32" drill bit — 0.1563 in, 3.97 mm
- #22 drill bit — 0.157 in, 3.99 mm
- #21 drill bit — 0.159 in, 4.04 mm
- #20 drill bit — 0.161 in, 4.09 mm