A letter i drill measures 0.2720 inch (6.909 mm). The closest fractional drill is 17/64" (0.2656") and the closest metric drill is 6.9 mm (0.2717"). It is the standard tap drill for 5/16-24.
What are the letter i drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| I | letter | 0.2720 | 6.909 | 0 |
| 6.9 mm | metric | 0.2717 | 6.900 | -0.0003 |
Tap drill for: 5/16-24.
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 i drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 2,809 | 0.0054 |
| 360 free-machining brass | 2,106 | 0.0049 |
| 1018 cold rolled steel | 1,123 | 0.0041 |
| 304 stainless steel | 562 | 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 i drill in inches?
0.2720 inch, which is 6.909 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, 6.9 mm is within 0.0003 inch.
What tap uses a letter i drill?
It is the standard 75% thread tap drill for 5/16-24. Tap drills are chosen to leave about 75% of full thread depth, the balance of strength and tapping torque that published charts use.