---
type: "drill"
designation: "#21"
series: "number"
series_name: "Number Drill Sizes"
diameter_in: 0.159
diameter_mm: 4.0386
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/number/21
---

# #21 Drill Bit Size — 0.1590 in / 4.039 mm

A #21 drill bit is 0.1590 in (4.039 mm) in diameter. It belongs to the number series of standard twist drills. It is the recommended tap drill for #10-32 UNF.

- **Designation**: #21
- **Series**: Number
- **Diameter**: 0.1590 in (4.039 mm)

## #21 Drill Diameter

| Property | Value |
| --- | --- |
| Designation | #21 |
| Series | Number |
| Diameter (in) | 0.1590 |
| Diameter (mm) | 4.039 |

## #21 Drill Equivalents in the Other Size Series

The nearest standard drill to #21 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 |
| --- | --- | --- | --- | --- |
| Letter | [A](https://shopmath.org/drills/letter/a.md) | 0.2340 | 5.944 | +0.0750 in |
| Fractional | [5/32](https://shopmath.org/drills/fractional/5-32.md) | 0.1563 | 3.969 | -0.0028 in |
| Metric | [4 mm](https://shopmath.org/drills/metric/4mm.md) | 0.1575 | 4.000 | -0.0015 in |

## Threads Tapped With a #21 Drill

| Thread | Pitch | Thread engagement (%) |
| --- | --- | --- |
| [#10-32 UNF](https://shopmath.org/threads/unf/10-32.md) | 32 TPI | 76.4 |

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

## Sources and Method for the #21 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 #21 Drill Bit

### What size is a #21 drill bit size?

A #21 drill bit is 0.1590 in (4.039 mm) in diameter.

### What is the metric equivalent of a #21 drill bit?

The closest metric drill to #21 (0.1590 in (4.039 mm)) is 4 mm at 0.1575 in (4.000 mm) — a difference of 0.0015 in.

### What fraction is a #21 drill bit?

#21 is 0.1590 in (4.039 mm), which is not itself a fractional size. The closest fractional drill is 5/32 at 0.1563 in (3.969 mm).

### What thread does a #21 drill bit tap?

A #21 drill is the recommended tap drill for #10-32 UNF. That leaves the thread engagement shown in the table above with a cut tap.

## Related pages

- Next size down (any series): [4 mm](https://shopmath.org/drills/metric/4mm.md) — 0.1575 in (4.000 mm)
- Next size up (any series): [#20](https://shopmath.org/drills/number/20.md) — 0.1610 in (4.089 mm)
- Previous in the number series: [#20](https://shopmath.org/drills/number/20.md)
- Next in the number series: [#22](https://shopmath.org/drills/number/22.md)
- All number sizes: [Number drill sizes](https://shopmath.org/drills/number.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/number/21) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*