A letter w drill measures 0.3860 inch (9.804 mm). The closest fractional drill is 25/64" (0.3906") and the closest metric drill is 9.8 mm (0.3858").
What are the letter w drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| W | letter | 0.3860 | 9.804 | 0 |
| 9.8 mm | metric | 0.3858 | 9.800 | -0.0002 |
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 letter w drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 1,979 | 0.0077 |
| 360 free-machining brass | 1,484 | 0.0069 |
| 1018 cold rolled steel | 792 | 0.0058 |
| 304 stainless steel | 396 | 0.0039 |
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 letter w drill in inches?
0.3860 inch, which is 9.804 mm. Letter drills run from A (0.234 inch) to Z (0.413 inch), continuing upward from the number drills. If you do not have this exact size, 9.8 mm is within 0.0002 inch.
What tap uses a letter w 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.