# 5/16" drill size

A 5/16 in drill is 0.3125 in (7.94 mm). It is the standard tap drill for 3/8-16 UNC; the nearest metric drill is 7.9 mm.

## 5/16" drill data

- Series: Fractional drill
- Decimal: 0.3125 in
- Metric: 7.94 mm (7.9375 mm exact)
- Nearest metric drill: 7.9 mm (−0.037 mm)
- Nearest letter drill: Letter O (0.3160 in)
- Standard tap drill for: 3/8-16 UNC

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 7.9 mm | 0.3110 | −0.037 mm |
| 8.0 mm | 0.3150 | +0.062 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| 19/64" | Fractional | 0.2969 | 7.54 |
| Letter N | Letter | 0.3020 | 7.67 |
| 5/16" | Fractional | 0.3125 | 7.94 |
| Letter O | Letter | 0.3160 | 8.03 |
| 21/64" | Fractional | 0.3281 | 8.33 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| 3/8-16 UNC | 5/16 | 77 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| 1/4" bolt | 0.2500" | 0.0625" | 0.0312" | – |


## FAQ

**What size is a 5/16 in drill?**
A 5/16 in drill is 0.3125 in, which is 7.94 mm (7.9375 mm exactly).

**What is the metric equivalent of a 5/16 in drill?**
The nearest metric drill is 7.9 mm, 0.037 mm smaller. The other close size is 8.0 mm.

**What tap uses a 5/16 in drill?**
A 5/16 in drill is the standard tap drill for 3/8-16 UNC (about 77 % thread).

**Which number or letter drill is closest to 5/16 in?**
Letter O (0.3160 in) is the closest, 0.0035 in larger.

**What does a 5/16 in drill clear?**
It clears a 1/4" bolt with 0.0625" total clearance (0.0312" per side).

Source page: https://machinistcharts.com/drill-size/5-16-inch/
Updated: 2026-10-06
Publisher: MachinistCharts (https://machinistcharts.com)