# 3/8-16 tap drill size

The tap drill for a 3/8-16 UNC thread is a 5/16 drill (0.3125 in, 7.94 mm). It leaves about 77 % thread. The closest metric drill is 7.9 mm, which leaves 79 % thread.

## Drill options for 3/8-16 UNC

| Drill | Inch | Thread % |
|---|---|---|
| 5/16 | 0.3125 | 77 % |
| 7.8 mm | 0.3071 | 84 % |
| 7.9 mm | 0.3110 | 79 % |
| 8.0 mm | 0.3150 | 74 % |

## 3/8-16 thread data

- Thread: 3/8-16 UNC, 16 TPI
- Major diameter: 0.3750 in (9.52 mm)
- Tap drill: 5/16 = 0.3125 in (7.94 mm)
- Thread engagement: 77 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 16 × 0.0625 / 0.01299 = 77 %.

## FAQ

**What size drill for a 3/8-16 tap?**
Use a 5/16 drill (0.3125 in, 7.94 mm). It leaves about 77 % thread.

**Can I use a metric drill for 3/8-16?**
Yes. A 7.9 mm drill gives about 79 % thread.

**How tight should a 3/8 grade 5 bolt be?**
About 31 ft-lb dry for 3/8-16 UNC.

Source page: https://machinistcharts.com/tap-drill/3-8-16/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)