# 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" drill data

- Series: Fractional drill
- Decimal: 0.3281 in
- Metric: 8.33 mm (8.3344 mm exact)
- Nearest metric drill: 8.3 mm (−0.034 mm)
- Nearest letter drill: Letter Q (0.3320 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 8.3 mm | 0.3268 | −0.034 mm |
| 8.4 mm | 0.3307 | +0.066 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| 5/16" | Fractional | 0.3125 | 7.94 |
| Letter P | Letter | 0.3230 | 8.20 |
| 21/64" | Fractional | 0.3281 | 8.33 |
| Letter Q | Letter | 0.3320 | 8.43 |
| 11/32" | Fractional | 0.3438 | 8.73 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| 3/8-16 UNC | 5/16 | 58 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| 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) |


## FAQ

**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.

Source page: https://machinistcharts.com/drill-size/21-64-inch/
Updated: 2026-10-06
Publisher: MachinistCharts (https://machinistcharts.com)