# 1-8 tap drill size

The tap drill for a 1-8 UNC thread is a 7/8 drill (0.8750 in, 22.22 mm). It leaves about 77 % thread. The closest metric drill is 22.2 mm, which leaves 78 % thread.

## Drill options for 1-8 UNC

| Drill | Inch | Thread % |
|---|---|---|
| 7/8 | 0.8750 | 77 % |
| 22.1 mm | 0.8701 | 80 % |
| 22.2 mm | 0.8740 | 78 % |
| 22.3 mm | 0.8780 | 75 % |

## 1-8 thread data

- Thread: 1-8 UNC, 8 TPI
- Major diameter: 1.0000 in (25.40 mm)
- Tap drill: 7/8 = 0.8750 in (22.22 mm)
- Thread engagement: 77 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 8 × 0.1250 / 0.01299 = 77 %.

## FAQ

**What size drill for a 1-8 tap?**
Use a 7/8 drill (0.8750 in, 22.22 mm). It leaves about 77 % thread.

**Can I use a metric drill for 1-8?**
Yes. A 22.2 mm drill gives about 78 % thread.

**How tight should a 1 grade 5 bolt be?**
About 644 ft-lb dry for 1-8 UNC.

Source page: https://machinistcharts.com/tap-drill/1-8/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)