# Letter U drill size

A letter U drill is 0.3680 in (9.35 mm). It is the standard tap drill for 7/16-14 UNC; the nearest metric drill is 9.3 mm.

## Letter U drill data

- Series: Letter drill
- Decimal: 0.3680 in
- Metric: 9.35 mm (9.3472 mm exact)
- Nearest metric drill: 9.3 mm (−0.047 mm)
- Nearest fractional: 3/8" (0.3750 in)
- Standard tap drill for: 7/16-14 UNC

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 9.3 mm | 0.3661 | −0.047 mm |
| 9.4 mm | 0.3701 | +0.053 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| Letter T | Letter | 0.3580 | 9.09 |
| 23/64" | Fractional | 0.3594 | 9.13 |
| Letter U | Letter | 0.3680 | 9.35 |
| 3/8" | Fractional | 0.3750 | 9.52 |
| Letter V | Letter | 0.3770 | 9.58 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| 7/16-14 UNC | U | 75 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| 5/16" bolt | 0.3125" | 0.0555" | 0.0277" | – |
| M8 bolt | 8 mm | 1.35 mm | 0.67 mm | Normal fit (normal 9, loose 10 mm) |


## FAQ

**What size is a letter U drill?**
A letter U drill is 0.3680 in, which is 9.35 mm (9.3472 mm exactly).

**What is the metric equivalent of a letter U drill?**
The nearest metric drill is 9.3 mm, 0.047 mm smaller. The other close size is 9.4 mm.

**What tap uses a letter U drill?**
A letter U drill is the standard tap drill for 7/16-14 UNC (about 75 % thread).

**Which fractional drill is closest to letter U?**
3/8 in (0.3750 in) is the closest, 0.0070 in larger.

**What does a letter U drill clear?**
It clears a M8 bolt with 1.35 mm total clearance (0.67 mm per side); for metric bolts: normal fit.

Source page: https://machinistcharts.com/drill-size/letter-u/
Updated: 2026-10-06
Publisher: MachinistCharts (https://machinistcharts.com)