A letter h drill measures 0.2660 inch (6.756 mm). The closest fractional drill is 17/64" (0.2656") and the closest metric drill is 6.8 mm (0.2677").
What are the letter h drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| H | letter | 0.2660 | 6.756 | 0 |
| 6.7 mm | metric | 0.2638 | 6.700 | -0.0022 |
| 17/64" | fraction | 0.2656 | 6.747 | -0.0004 |
| 6.8 mm | metric | 0.2677 | 6.800 | +0.0017 |
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 h drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 2,872 | 0.0053 |
| 360 free-machining brass | 2,154 | 0.0048 |
| 1018 cold rolled steel | 1,149 | 0.0040 |
| 304 stainless steel | 574 | 0.0027 |
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 h drill in inches?
0.2660 inch, which is 6.756 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, 6.8 mm is within 0.0017 inch.
What tap uses a letter h 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.