21/64" drill size
A 21/64 in drill is 0.3281 in (8.33 mm). It sits between 5/16 in and 11/32 in in the fractional series; the nearest metric drill is 8.3 mm.
21/64" 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 |
|---|---|---|---|---|
| 8.3 mm | 0.3268" | −0.034 mm | −0.0014" | Smaller |
| 8.4 mm | 0.3307" | +0.066 mm | +0.0026" | 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 | −0.1001" |
| 5/16" | Fractional | 0.3125" | 7.94 mm | −0.0156" |
| Letter P | Letter | 0.3230" | 8.20 mm | −0.0051" |
| 21/64" | Fractional | 0.3281" | 8.33 mm | this size |
| Letter Q | Letter | 0.3320" | 8.43 mm | +0.0039" |
| 11/32" | Fractional | 0.3438" | 8.73 mm | +0.0156" |
Threads a 21/64" 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 |
|---|---|---|---|
| 3/8-16 UNC | 5/16 | 58 % | Hard, tough metals |
Clearance for screws and bolts
Bolts and screws a 21/64" 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 |
|---|---|---|---|---|
| 5/16" bolt | 0.3125" | 0.0156" | 0.0078" | – |
| M8 bolt | 8 mm | 0.33 mm | 0.17 mm | Tighter than close fit (8.4 mm) |
Inch clearance is the drill size minus the basic major diameter. A drilled hole usually cuts slightly oversize.
Fractional drill index
Every size in the series, holes drawn to scale.
Common questions
What size is a 21/64 in drill?
A 21/64 in drill is 0.3281 in, which is 8.33 mm (8.3344 mm exactly).
What is the metric equivalent of a 21/64 in drill?
The nearest metric drill is 8.3 mm, 0.034 mm smaller. The other close size is 8.4 mm.
What thread can I tap with a 21/64 in drill?
It leaves about 58 % thread in 3/8-16 UNC.
Which number or letter drill is closest to 21/64 in?
Letter Q (0.3320 in) is the closest, 0.0039 in larger.
What does a 21/64 in drill clear?
It clears a M8 bolt with 0.33 mm total clearance (0.17 mm per side); for metric bolts: tighter than close fit.