Letter I drill size
A letter I drill is 0.2720 in (6.91 mm). It is the standard tap drill for 5/16-24 UNF; the nearest metric drill is 6.9 mm.
Letter I at a glance
Decimal, metric and the nearest sizes in the other series.
- Decimal inch
- 0.2720"
- Millimetres
- 6.91 mm 6.9088
- Nearest metric drill
- 6.9 mm −0.009 mm
- Nearest fractional
- 17/64" 0.2656", −0.0064"
- Hole area
- 0.0581 in² 37.49 mm²
- Tap drill for
- 5/16-24 UNF
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 |
|---|---|---|---|---|
| 6.9 mm | 0.2717" | −0.009 mm | −0.0003" | Smaller |
| 7.0 mm | 0.2756" | +0.091 mm | +0.0036" | 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 | −0.0440" |
| 17/64" | Fractional | 0.2656" | 6.75 mm | −0.0064" |
| Letter H | Letter | 0.2660" | 6.76 mm | −0.0060" |
| Letter I | Letter | 0.2720" | 6.91 mm | this size |
| Letter J | Letter | 0.2770" | 7.04 mm | +0.0050" |
| 9/32" | Fractional | 0.2812" | 7.14 mm | +0.0092" |
Threads a Letter I 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 |
|---|---|---|---|
| 5/16-24 UNF | I | 75 % | Standard drill |
| M8 × 1.25 | 6.8 mm | 67 % | Good in steel |
| M8 × 1 | 7 mm | 84 % | Soft metals, plastics |
| 5/16-18 UNC | F | 56 % | Hard, tough metals |
Clearance for screws and bolts
Bolts and screws a Letter I 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" | 0.0560" | 0.0280" | – |
| M6 bolt | 6 mm | 0.91 mm | 0.45 mm | Normal fit (normal 6.6, loose 7 mm) |
| 1/4" bolt | 0.2500" | 0.0220" | 0.0110" | – |
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 I drill?
A letter I drill is 0.2720 in, which is 6.91 mm (6.9088 mm exactly).
What is the metric equivalent of a letter I drill?
The nearest metric drill is 6.9 mm, 0.009 mm smaller. The other close size is 7.0 mm.
What tap uses a letter I drill?
A letter I drill is the standard tap drill for 5/16-24 UNF (about 75 % thread).
Which fractional drill is closest to letter I?
17/64 in (0.2656 in) is the closest, 0.0064 in smaller.
What does a letter I drill clear?
It clears a 1/4" bolt with 0.0220" total clearance (0.0110" per side).