---
type: "drill"
designation: "25/64"
series: "fractional"
series_name: "Fractional Drill Sizes"
diameter_in: 0.390625
diameter_mm: 9.921875
fraction_label: "25/64"
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/fractional/25-64
---

# 25/64 Drill Bit Size — 0.3906 in / 9.922 mm

A 25/64 drill bit is 0.3906 in (9.922 mm) in diameter. It belongs to the fractional series of standard twist drills. It is the recommended tap drill for 7/16"-20 UNF.

- **Designation**: 25/64
- **Series**: Fractional
- **Diameter**: 0.3906 in (9.922 mm)
- **Fraction**: 25/64

## 25/64 Drill Diameter

| Property | Value |
| --- | --- |
| Designation | 25/64 |
| Series | Fractional |
| Diameter (in) | 0.3906 |
| Diameter (mm) | 9.922 |
| Fractional call-out | 25/64 |

## 25/64 Drill Equivalents in the Other Size Series

The nearest standard drill to 25/64 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 | -0.1626 in |
| Letter | [W](https://shopmath.org/drills/letter/w.md) | 0.3860 | 9.804 | -0.0046 in |
| Metric | [9.9 mm](https://shopmath.org/drills/metric/9-9mm.md) | 0.3898 | 9.900 | -0.0009 in |

## Threads Tapped With a 25/64 Drill

| Thread | Pitch | Thread engagement (%) |
| --- | --- | --- |
| [7/16"-20 UNF](https://shopmath.org/threads/unf/7-16-20.md) | 20 TPI | 72.2 |

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

## Sources and Method for the 25/64 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**: computed
- **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 25/64 Drill Bit

### What size is a 25/64 drill bit size?

A 25/64 drill bit is 0.3906 in (9.922 mm) in diameter. The nearest fractional call-out is 25/64.

### What is the metric equivalent of a 25/64 drill bit?

The closest metric drill to 25/64 (0.3906 in (9.922 mm)) is 9.9 mm at 0.3898 in (9.900 mm) — a difference of 0.0009 in.

### What thread does a 25/64 drill bit tap?

A 25/64 drill is the recommended tap drill for 7/16"-20 UNF. That leaves the thread engagement shown in the table above with a cut tap.

## Related pages

- Next size down (any series): [9.9 mm](https://shopmath.org/drills/metric/9-9mm.md) — 0.3898 in (9.900 mm)
- Next size up (any series): [10 mm](https://shopmath.org/drills/metric/10mm.md) — 0.3937 in (10.000 mm)
- Previous in the fractional series: [3/8](https://shopmath.org/drills/fractional/3-8.md)
- Next in the fractional series: [13/32](https://shopmath.org/drills/fractional/13-32.md)
- All fractional sizes: [Fractional drill sizes](https://shopmath.org/drills/fractional.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/fractional/25-64) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*