A letter g drill measures 0.2610 inch (6.629 mm). The closest fractional drill is 17/64" (0.2656") and the closest metric drill is 6.6 mm (0.2598").
What are the letter g drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| G | letter | 0.2610 | 6.629 | 0 |
| 6.6 mm | metric | 0.2598 | 6.600 | -0.0012 |
| 6.7 mm | metric | 0.2638 | 6.700 | +0.0028 |
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 g drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 2,927 | 0.0052 |
| 360 free-machining brass | 2,195 | 0.0047 |
| 1018 cold rolled steel | 1,171 | 0.0039 |
| 304 stainless steel | 585 | 0.0026 |
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 g drill in inches?
0.2610 inch, which is 6.629 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, 6.6 mm is within 0.0012 inch.
What tap uses a letter g 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.