# 1/2-20 tap drill size

The tap drill for a 1/2-20 UNF thread is a 29/64 drill (0.4531 in, 11.51 mm). It leaves about 72 % thread. The closest metric drill is 11.5 mm, which leaves 73 % thread.

## Drill options for 1/2-20 UNF

| Drill | Inch | Thread % |
|---|---|---|
| 29/64 | 0.4531 | 72 % |
| 11.4 mm | 0.4488 | 79 % |
| 11.5 mm | 0.4528 | 73 % |
| 11.6 mm | 0.4567 | 67 % |

## 1/2-20 thread data

- Thread: 1/2-20 UNF, 20 TPI
- Major diameter: 0.5000 in (12.70 mm)
- Tap drill: 29/64 = 0.4531 in (11.51 mm)
- Thread engagement: 72 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 20 × 0.0469 / 0.01299 = 72 %.

## FAQ

**What size drill for a 1/2-20 tap?**
Use a 29/64 drill (0.4531 in, 11.51 mm). It leaves about 72 % thread.

**Can I use a metric drill for 1/2-20?**
Yes. A 11.5 mm drill gives about 73 % thread.

**How tight should a 1/2 grade 5 bolt be?**
About 85 ft-lb dry for 1/2-20 UNF.

Source page: https://machinistcharts.com/tap-drill/1-2-20/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)