---
type: "drill"
designation: "5/16"
series: "fractional"
series_name: "Fractional Drill Sizes"
diameter_in: 0.3125
diameter_mm: 7.9375
fraction_label: "5/16"
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/fractional/5-16
---

# 5/16 Drill Bit Size — 0.3125 in / 7.938 mm

A 5/16 drill bit is 0.3125 in (7.938 mm) in diameter. It belongs to the fractional series of standard twist drills. It is the recommended tap drill for 3/8"-16 UNC.

- **Designation**: 5/16
- **Series**: Fractional
- **Diameter**: 0.3125 in (7.938 mm)
- **Fraction**: 5/16

## 5/16 Drill Diameter

| Property | Value |
| --- | --- |
| Designation | 5/16 |
| Series | Fractional |
| Diameter (in) | 0.3125 |
| Diameter (mm) | 7.938 |
| Fractional call-out | 5/16 |

## 5/16 Drill Equivalents in the Other Size Series

The nearest standard drill to 5/16 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 | -0.0845 in |
| Letter | [O](https://shopmath.org/drills/letter/o.md) | 0.3160 | 8.026 | +0.0035 in |
| Metric | [7.9 mm](https://shopmath.org/drills/metric/7-9mm.md) | 0.3110 | 7.900 | -0.0015 in |

## Threads Tapped With a 5/16 Drill

| Thread | Pitch | Thread engagement (%) |
| --- | --- | --- |
| [3/8"-16 UNC](https://shopmath.org/threads/unc/3-8-16.md) | 16 TPI | 77.0 |

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

## Sources and Method for the 5/16 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 5/16 Drill Bit

### What size is a 5/16 drill bit size?

A 5/16 drill bit is 0.3125 in (7.938 mm) in diameter. The nearest fractional call-out is 5/16.

### What is the metric equivalent of a 5/16 drill bit?

The closest metric drill to 5/16 (0.3125 in (7.938 mm)) is 7.9 mm at 0.3110 in (7.900 mm) — a difference of 0.0015 in.

### What thread does a 5/16 drill bit tap?

A 5/16 drill is the recommended tap drill for 3/8"-16 UNC. That leaves the thread engagement shown in the table above with a cut tap.

## Related pages

- Next size down (any series): [7.9 mm](https://shopmath.org/drills/metric/7-9mm.md) — 0.3110 in (7.900 mm)
- Next size up (any series): [8 mm](https://shopmath.org/drills/metric/8mm.md) — 0.3150 in (8.000 mm)
- Previous in the fractional series: [19/64](https://shopmath.org/drills/fractional/19-64.md)
- Next in the fractional series: [21/64](https://shopmath.org/drills/fractional/21-64.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/5-16) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*