A #18 drill measures 0.1695 inch (4.305 mm). The closest fractional drill is 11/64" (0.1719") and the closest metric drill is 4.3 mm (0.1693").
What are the #18 drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| #18 | number | 0.1695 | 4.305 | 0 |
| 4.3 mm | metric | 0.1693 | 4.300 | -0.0002 |
| 11/64" | fraction | 0.1719 | 4.366 | +0.0024 |
Drill sets that include this size
Number and letter sizes come in indexed sets; buy the set rather than single drills.
What speed should a #18 drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 4,507 | 0.0034 |
| 360 free-machining brass | 3,380 | 0.0031 |
| 1018 cold rolled steel | 1,803 | 0.0025 |
| 304 stainless steel | 901 | 0.0017 |
Starting points from conservative HSS drilling speeds. Small drills often call for more RPM than a benchtop drill press offers; run at the top speed and feed gently. Use cutting oil in steel and stainless, and clamp the work: never hold it by hand.
Cutting fluid for drilling
Frequently asked questions
What is a #18 drill in inches?
0.1695 inch, which is 4.305 mm. Number drills get smaller as the number rises, so the next number up is a smaller drill. If you do not have this exact size, 4.3 mm is within 0.0002 inch.
What tap uses a #18 drill?
No common UNC or UNF thread uses it as its standard 75% tap drill, but it often serves as an alternative drill for a slightly different percentage of thread, or as a clearance or pilot size. The tap drill chart lists every standard pairing.