#18 drill size
A #18 drill is 0.1695 in (4.31 mm). It sits between #19 and #17 in the number series; the nearest metric drill is 4.3 mm.
#18 at a glance
Decimal, metric and the nearest sizes in the other series.
Nearest metric drills
Metric drills step by 0.05 mm below 3 mm and by 0.1 mm from 3 mm. Difference is metric minus inch size.
| Metric drill | Inch | Difference | In inches | Hole |
|---|---|---|---|---|
| 4.3 mm | 0.1693" | −0.005 mm | −0.0002" | Smaller |
| 4.4 mm | 0.1732" | +0.095 mm | +0.0037" | Larger |
Nearby inch drills
The next size down and up in each inch series, so you can pick a substitute from the drawer.
Threads a #18 drill can tap
Threads from the tap tables on this site that this drill can tap, with the thread percentage it leaves (55–85 % shown).
| Thread | Standard drill | Thread with this drill | Suits |
|---|---|---|---|
| M5 × 0.8 | 4.2 mm | 67 % | Good in steel |
Clearance for screws and bolts
Bolts and screws a #18 hole clears, with the diametral clearance and the gap each side. Metric sizes are compared with ISO 273.
| Fastener | Diameter | Clearance | Per side | ISO 273, metric only |
|---|---|---|---|---|
| #4 screw | 0.1120" | 0.0575" | 0.0288" | – |
| #5 screw | 0.1250" | 0.0445" | 0.0223" | – |
| #6 screw | 0.1380" | 0.0315" | 0.0158" | – |
| M4 bolt | 4 mm | 0.31 mm | 0.15 mm | Close fit (close 4.3, normal 4.5 mm) |
| #8 screw | 0.1640" | 0.0055" | 0.0028" | – |
Inch clearance is the drill size minus the basic major diameter. A drilled hole usually cuts slightly oversize.
Number drill index
Every size in the series, holes drawn to scale.
Common questions
What size is a #18 drill?
A #18 drill is 0.1695 in, which is 4.31 mm (4.3053 mm exactly).
What is the metric equivalent of a #18 drill?
The nearest metric drill is 4.3 mm, 0.005 mm smaller. The other close size is 4.4 mm.
What thread can I tap with a #18 drill?
It leaves about 67 % thread in M5 × 0.8.
Which fractional drill is closest to #18?
11/64 in (0.1719 in) is the closest, 0.0024 in larger.
What does a #18 drill clear?
It clears a #8 screw with 0.0055" total clearance (0.0028" per side).