---
type: "drill"
designation: "31/64"
series: "fractional"
series_name: "Fractional Drill Sizes"
diameter_in: 0.484375
diameter_mm: 12.303125
fraction_label: "31/64"
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/fractional/31-64
---

# 31/64 Drill Bit Size — 0.4844 in / 12.303 mm

A 31/64 drill bit is 0.4844 in (12.303 mm) in diameter. It belongs to the fractional series of standard twist drills. It is the recommended tap drill for 9/16"-12 UNC.

- **Designation**: 31/64
- **Series**: Fractional
- **Diameter**: 0.4844 in (12.303 mm)
- **Fraction**: 31/64

## 31/64 Drill Diameter

| Property | Value |
| --- | --- |
| Designation | 31/64 |
| Series | Fractional |
| Diameter (in) | 0.4844 |
| Diameter (mm) | 12.303 |
| Fractional call-out | 31/64 |

## 31/64 Drill Equivalents in the Other Size Series

The nearest standard drill to 31/64 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 | -0.2564 in |
| Letter | [Z](https://shopmath.org/drills/letter/z.md) | 0.4130 | 10.490 | -0.0714 in |
| Metric | [12.3 mm](https://shopmath.org/drills/metric/12-3mm.md) | 0.4843 | 12.300 | -0.0001 in |

## Threads Tapped With a 31/64 Drill

| Thread | Pitch | Thread engagement (%) |
| --- | --- | --- |
| [9/16"-12 UNC](https://shopmath.org/threads/unc/9-16-12.md) | 12 TPI | 72.2 |

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

## Sources and Method for the 31/64 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 31/64 Drill Bit

### What size is a 31/64 drill bit size?

A 31/64 drill bit is 0.4844 in (12.303 mm) in diameter. The nearest fractional call-out is 31/64.

### What is the metric equivalent of a 31/64 drill bit?

The closest metric drill to 31/64 (0.4844 in (12.303 mm)) is 12.3 mm at 0.4843 in (12.300 mm) — a difference of 0.0001 in.

### What thread does a 31/64 drill bit tap?

A 31/64 drill is the recommended tap drill for 9/16"-12 UNC. That leaves the thread engagement shown in the table above with a cut tap.

## Related pages

- Next size down (any series): [12.3 mm](https://shopmath.org/drills/metric/12-3mm.md) — 0.4843 in (12.300 mm)
- Next size up (any series): [12.4 mm](https://shopmath.org/drills/metric/12-4mm.md) — 0.4882 in (12.400 mm)
- Previous in the fractional series: [15/32](https://shopmath.org/drills/fractional/15-32.md)
- Next in the fractional series: [1/2](https://shopmath.org/drills/fractional/1-2.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/31-64) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*