A letter k drill measures 0.2810 inch (7.137 mm). The closest fractional drill is 9/32" (0.2813") and the closest metric drill is 7.1 mm (0.2795").
What are the letter k drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| K | letter | 0.2810 | 7.137 | 0 |
| 7.1 mm | metric | 0.2795 | 7.100 | -0.0015 |
| 9/32" | fraction | 0.2813 | 7.144 | +0.0003 |
| 7.2 mm | metric | 0.2835 | 7.200 | +0.0025 |
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 k drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 2,719 | 0.0056 |
| 360 free-machining brass | 2,039 | 0.0051 |
| 1018 cold rolled steel | 1,087 | 0.0042 |
| 304 stainless steel | 544 | 0.0028 |
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 k drill in inches?
0.2810 inch, which is 7.137 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, 7.1 mm is within 0.0015 inch.
What tap uses a letter k 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.