Letter H drill size
A letter H drill is 0.2660 in (6.76 mm). It sits between letter G and letter I in the letter series; the nearest metric drill is 6.8 mm.
Letter H at a glance
Decimal, metric and the nearest sizes in the other series.
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 |
|---|---|---|---|---|
| 6.7 mm | 0.2638" | −0.056 mm | −0.0022" | Smaller |
| 6.8 mm | 0.2677" | +0.044 mm | +0.0017" | 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 | −0.0380" |
| Letter G | Letter | 0.2610" | 6.63 mm | −0.0050" |
| 17/64" | Fractional | 0.2656" | 6.75 mm | −0.0004" |
| Letter H | Letter | 0.2660" | 6.76 mm | this size |
| Letter I | Letter | 0.2720" | 6.91 mm | +0.0060" |
| 9/32" | Fractional | 0.2812" | 7.14 mm | +0.0152" |
Threads a Letter H 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 |
|---|---|---|---|
| M8 × 1.25 | 6.8 mm | 77 % | Soft metals, plastics |
| 5/16-18 UNC | F | 64 % | Hard, tough metals |
Clearance for screws and bolts
Bolts and screws a Letter H 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" | 0.0500" | 0.0250" | – |
| M6 bolt | 6 mm | 0.76 mm | 0.38 mm | Normal fit (normal 6.6, loose 7 mm) |
| 1/4" bolt | 0.2500" | 0.0160" | 0.0080" | – |
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 H drill?
A letter H drill is 0.2660 in, which is 6.76 mm (6.7564 mm exactly).
What is the metric equivalent of a letter H drill?
The nearest metric drill is 6.8 mm, 0.044 mm larger. The other close size is 6.7 mm.
What thread can I tap with a letter H drill?
It leaves about 77 % thread in M8 × 1.25 and 64 % in 5/16-18 UNC.
Which fractional drill is closest to letter H?
17/64 in (0.2656 in) is the closest, 0.0004 in smaller.
What does a letter H drill clear?
It clears a 1/4" bolt with 0.0160" total clearance (0.0080" per side).