---
type: "drill"
designation: "U"
series: "letter"
series_name: "Letter Drill Sizes"
diameter_in: 0.368
diameter_mm: 9.3472
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/letter/u
---

# U Drill Bit Size — 0.3680 in / 9.347 mm

A U drill bit is 0.3680 in (9.347 mm) in diameter. It belongs to the letter series of standard twist drills. It is the recommended tap drill for 7/16"-14 UNC.

- **Designation**: U
- **Series**: Letter
- **Diameter**: 0.3680 in (9.347 mm)

## U Drill Diameter

| Property | Value |
| --- | --- |
| Designation | U |
| Series | Letter |
| Diameter (in) | 0.3680 |
| Diameter (mm) | 9.347 |

## U Drill Equivalents in the Other Size Series

The nearest standard drill to U in each of the other three series. Nothing here is an exact match unless the difference column reads 0.0000 in.

| Series | Nearest drill | Diameter (in) | Diameter (mm) | Difference |
| --- | --- | --- | --- | --- |
| Number | [#1](https://shopmath.org/drills/number/1.md) | 0.2280 | 5.791 | -0.1400 in |
| Fractional | [3/8](https://shopmath.org/drills/fractional/3-8.md) | 0.3750 | 9.525 | +0.0070 in |
| Metric | [9.3 mm](https://shopmath.org/drills/metric/9-3mm.md) | 0.3661 | 9.300 | -0.0019 in |

## Threads Tapped With a U Drill

| Thread | Pitch | Thread engagement (%) |
| --- | --- | --- |
| [7/16"-14 UNC](https://shopmath.org/threads/unc/7-16-14.md) | 14 TPI | 74.9 |

_Engagement is what the real drill leaves with a cut tap, not the requested target._

## Sources and Method for the U Drill Size

### Drill index

- **Source**: Number/letter: bd_warehouse drill_sizes.csv (Apache-2.0) cross-checked against Wikipedia Drill bit sizes (CC BY-SA 4.0). Fractional: computed 1/64 in steps per ANSI B94.11M. Metric: computed BS 328 increment pattern.
- **Method**: compiled
- **Verified against**: Wikipedia Drill bit sizes (CC BY-SA 4.0); 31/31 checks passed

### Assumptions

- Drill diameters are nominal. A worn or run-out drill cuts oversize; measure the hole, not the shank, when the fit matters.
- Dataset scope: Number sizes #61-#80 are stored but tagged demandTier 'extended' — query-space-map.md Appendix A found the tail thinning in the 60s. Sizes below #80 and metric sizes above 25.0 mm excluded.

**Data last verified**: August 1, 2026

## Common Questions About the U Drill Bit

### What size is a U drill bit size?

A U drill bit is 0.3680 in (9.347 mm) in diameter.

### What is the metric equivalent of a U drill bit?

The closest metric drill to U (0.3680 in (9.347 mm)) is 9.3 mm at 0.3661 in (9.300 mm) — a difference of 0.0019 in.

### What fraction is a U drill bit?

U is 0.3680 in (9.347 mm), which is not itself a fractional size. The closest fractional drill is 3/8 at 0.3750 in (9.525 mm).

### What thread does a U drill bit tap?

A U drill is the recommended tap drill for 7/16"-14 UNC. That leaves the thread engagement shown in the table above with a cut tap.

## Related pages

- Next size down (any series): [9.3 mm](https://shopmath.org/drills/metric/9-3mm.md) — 0.3661 in (9.300 mm)
- Next size up (any series): [9.4 mm](https://shopmath.org/drills/metric/9-4mm.md) — 0.3701 in (9.400 mm)
- Previous in the letter series: [T](https://shopmath.org/drills/letter/t.md)
- Next in the letter series: [V](https://shopmath.org/drills/letter/v.md)
- All letter sizes: [Letter drill sizes](https://shopmath.org/drills/letter.md)
- All drill sizes: [Drill bit size reference](https://shopmath.org/drills.md)
- Complete index by diameter: [Drill size chart](https://shopmath.org/charts/drill-sizes.md)
- Tap drills by thread: [Tap drill chart](https://shopmath.org/charts/tap-drill.md)

---

*Source: [ShopMath](https://shopmath.org/drills/letter/u) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*