# 1/2-13 tap drill size

The tap drill for a 1/2-13 UNC thread is a 27/64 drill (0.4219 in, 10.72 mm). It leaves about 78 % thread. The closest metric drill is 10.7 mm, which leaves 79 % thread.

## Drill options for 1/2-13 UNC

| Drill | Inch | Thread % |
|---|---|---|
| 27/64 | 0.4219 | 78 % |
| 10.6 mm | 0.4173 | 83 % |
| 10.7 mm | 0.4213 | 79 % |
| 10.8 mm | 0.4252 | 75 % |

## 1/2-13 thread data

- Thread: 1/2-13 UNC, 13 TPI
- Major diameter: 0.5000 in (12.70 mm)
- Tap drill: 27/64 = 0.4219 in (10.72 mm)
- Thread engagement: 78 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 13 × 0.0781 / 0.01299 = 78 %.

## FAQ

**What size drill for a 1/2-13 tap?**
Use a 27/64 drill (0.4219 in, 10.72 mm). It leaves about 78 % thread.

**Can I use a metric drill for 1/2-13?**
Yes. A 10.7 mm drill gives about 79 % thread.

**How tight should a 1/2 grade 5 bolt be?**
About 75 ft-lb dry for 1/2-13 UNC.

Source page: https://machinistcharts.com/tap-drill/1-2-13/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)