# 5/16-18 tap drill size

The tap drill for a 5/16-18 UNC thread is a F drill (0.2570 in, 6.53 mm). It leaves about 77 % thread. The closest metric drill is 6.5 mm, which leaves 78 % thread.

## Drill options for 5/16-18 UNC

| Drill | Inch | Thread % |
|---|---|---|
| F | 0.2570 | 77 % |
| 6.4 mm | 0.2520 | 84 % |
| 6.5 mm | 0.2559 | 78 % |
| 6.6 mm | 0.2598 | 73 % |

## 5/16-18 thread data

- Thread: 5/16-18 UNC, 18 TPI
- Major diameter: 0.3125 in (7.94 mm)
- Tap drill: F = 0.2570 in (6.53 mm)
- Thread engagement: 77 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 18 × 0.0555 / 0.01299 = 77 %.

## FAQ

**What size drill for a 5/16-18 tap?**
Use a F drill (0.2570 in, 6.53 mm). It leaves about 77 % thread.

**Can I use a metric drill for 5/16-18?**
Yes. A 6.5 mm drill gives about 78 % thread.

**How tight should a 5/16 grade 5 bolt be?**
About 17 ft-lb dry for 5/16-18 UNC.

Source page: https://machinistcharts.com/tap-drill/5-16-18/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)