A letter a drill measures 0.2340 inch (5.944 mm). The closest fractional drill is 15/64" (0.2344") and the closest metric drill is 5.9 mm (0.2323").
What are the letter a drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| A | letter | 0.2340 | 5.944 | 0 |
| 5.9 mm | metric | 0.2323 | 5.900 | -0.0017 |
| 15/64" | fraction | 0.2344 | 5.953 | +0.0004 |
| 6.0 mm | metric | 0.2362 | 6.000 | +0.0022 |
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 a drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 3,265 | 0.0047 |
| 360 free-machining brass | 2,449 | 0.0042 |
| 1018 cold rolled steel | 1,306 | 0.0035 |
| 304 stainless steel | 653 | 0.0023 |
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 a drill in inches?
0.2340 inch, which is 5.944 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, 5.9 mm is within 0.0017 inch.
What tap uses a letter a 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.