A letter c drill measures 0.2420 inch (6.147 mm). The closest fractional drill is 15/64" (0.2344") and the closest metric drill is 6.1 mm (0.2402").
What are the letter c drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| C | letter | 0.2420 | 6.147 | 0 |
| 6.1 mm | metric | 0.2402 | 6.100 | -0.0018 |
| 6.2 mm | metric | 0.2441 | 6.200 | +0.0021 |
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 c drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 3,157 | 0.0048 |
| 360 free-machining brass | 2,368 | 0.0044 |
| 1018 cold rolled steel | 1,263 | 0.0036 |
| 304 stainless steel | 631 | 0.0024 |
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 c drill in inches?
0.2420 inch, which is 6.147 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, 6.1 mm is within 0.0018 inch.
What tap uses a letter c 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.