A letter f drill measures 0.2570 inch (6.528 mm). The closest fractional drill is 1/4" (0.2500") and the closest metric drill is 6.5 mm (0.2559"). It is the standard tap drill for 5/16-18.
What are the letter f drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| F | letter | 0.2570 | 6.528 | 0 |
| 6.5 mm | metric | 0.2559 | 6.500 | -0.0011 |
| 6.6 mm | metric | 0.2598 | 6.600 | +0.0028 |
Tap drill for: 5/16-18.
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 f drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 2,973 | 0.0051 |
| 360 free-machining brass | 2,229 | 0.0046 |
| 1018 cold rolled steel | 1,189 | 0.0039 |
| 304 stainless steel | 595 | 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 f drill in inches?
0.2570 inch, which is 6.528 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, 6.5 mm is within 0.0011 inch.
What tap uses a letter f drill?
It is the standard 75% thread tap drill for 5/16-18. Tap drills are chosen to leave about 75% of full thread depth, the balance of strength and tapping torque that published charts use.