Letter F drill size
A letter F drill is 0.2570 in (6.53 mm). It is the standard tap drill for 5/16-18 UNC; the nearest metric drill is 6.5 mm.
Letter F at a glance
Decimal, metric and the nearest sizes in the other series.
- Decimal inch
- 0.2570"
- Millimetres
- 6.53 mm 6.5278
- Nearest metric drill
- 6.5 mm −0.028 mm
- Nearest fractional
- 1/4" 0.2500", −0.0070"
- Hole area
- 0.0519 in² 33.47 mm²
- Tap drill for
- 5/16-18 UNC
Nearest metric drills
Metric drills step by 0.05 mm below 3 mm and by 0.1 mm from 3 mm. Difference is metric minus inch size.
| Metric drill | Inch | Difference | In inches | Hole |
|---|---|---|---|---|
| 6.5 mm | 0.2559" | −0.028 mm | −0.0011" | Smaller |
| 6.6 mm | 0.2598" | +0.072 mm | +0.0028" | Larger |
Nearby inch drills
The next size down and up in each inch series, so you can pick a substitute from the drawer.
| Drill | Series | Inch | mm | Difference |
|---|---|---|---|---|
| #1 | Number | 0.2280" | 5.79 mm | −0.0290" |
| 1/4" | Fractional | 0.2500" | 6.35 mm | −0.0070" |
| Letter E | Letter | 0.2500" | 6.35 mm | −0.0070" |
| Letter F | Letter | 0.2570" | 6.53 mm | this size |
| Letter G | Letter | 0.2610" | 6.63 mm | +0.0040" |
| 17/64" | Fractional | 0.2656" | 6.75 mm | +0.0086" |
Threads a Letter F drill can tap
Threads from the tap tables on this site that this drill can tap, with the thread percentage it leaves (55–85 % shown).
| Thread | Standard drill | Thread with this drill | Suits |
|---|---|---|---|
| 5/16-18 UNC | F | 77 % | Standard drill |
Clearance for screws and bolts
Bolts and screws a Letter F hole clears, with the diametral clearance and the gap each side. Metric sizes are compared with ISO 273.
| Fastener | Diameter | Clearance | Per side | ISO 273, metric only |
|---|---|---|---|---|
| #12 screw | 0.2160" | 0.0410" | 0.0205" | – |
| M6 bolt | 6 mm | 0.53 mm | 0.26 mm | Close fit (close 6.4, normal 6.6 mm) |
| 1/4" bolt | 0.2500" | 0.0070" | 0.0035" | – |
Inch clearance is the drill size minus the basic major diameter. A drilled hole usually cuts slightly oversize.
Letter drill index
Every size in the series, holes drawn to scale.
Common questions
What size is a letter F drill?
A letter F drill is 0.2570 in, which is 6.53 mm (6.5278 mm exactly).
What is the metric equivalent of a letter F drill?
The nearest metric drill is 6.5 mm, 0.028 mm smaller. The other close size is 6.6 mm.
What tap uses a letter F drill?
A letter F drill is the standard tap drill for 5/16-18 UNC (about 77 % thread).
Which fractional drill is closest to letter F?
1/4 in (0.2500 in) is the closest, 0.0070 in smaller.
What does a letter F drill clear?
It clears a 1/4" bolt with 0.0070" total clearance (0.0035" per side).