---
type: "drill"
designation: "X"
series: "letter"
series_name: "Letter Drill Sizes"
diameter_in: 0.397
diameter_mm: 10.0838
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/letter/x
---

# X Drill Bit Size — 0.3970 in / 10.084 mm

A X drill bit is 0.3970 in (10.084 mm) in diameter. It belongs to the letter series of standard twist drills.

- **Designation**: X
- **Series**: Letter
- **Diameter**: 0.3970 in (10.084 mm)

## X Drill Diameter

| Property | Value |
| --- | --- |
| Designation | X |
| Series | Letter |
| Diameter (in) | 0.3970 |
| Diameter (mm) | 10.084 |

## X Drill Equivalents in the Other Size Series

The nearest standard drill to X 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 | -0.1690 in |
| Fractional | [25/64](https://shopmath.org/drills/fractional/25-64.md) | 0.3906 | 9.922 | -0.0064 in |
| Metric | [10.1 mm](https://shopmath.org/drills/metric/10-1mm.md) | 0.3976 | 10.100 | +0.0006 in |

## Sources and Method for the X 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 X Drill Bit

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

A X drill bit is 0.3970 in (10.084 mm) in diameter.

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

The closest metric drill to X (0.3970 in (10.084 mm)) is 10.1 mm at 0.3976 in (10.100 mm) — a difference of 0.0006 in.

### What fraction is a X drill bit?

X is 0.3970 in (10.084 mm), which is not itself a fractional size. The closest fractional drill is 25/64 at 0.3906 in (9.922 mm).

## Related pages

- Next size down (any series): [10 mm](https://shopmath.org/drills/metric/10mm.md) — 0.3937 in (10.000 mm)
- Next size up (any series): [10.1 mm](https://shopmath.org/drills/metric/10-1mm.md) — 0.3976 in (10.100 mm)
- Previous in the letter series: [W](https://shopmath.org/drills/letter/w.md)
- Next in the letter series: [Y](https://shopmath.org/drills/letter/y.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/x) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*