# 9/16-18 tap drill size

The tap drill for a 9/16-18 UNF thread is a 33/64 drill (0.5156 in, 13.10 mm). It leaves about 65 % thread. The closest metric drill is 13.1 mm, which leaves 65 % thread.

## Drill options for 9/16-18 UNF

| Drill | Inch | Thread % |
|---|---|---|
| 33/64 | 0.5156 | 65 % |
| 13.0 mm | 0.5118 | 70 % |
| 13.1 mm | 0.5158 | 65 % |
| 13.2 mm | 0.5197 | 59 % |

## 9/16-18 thread data

- Thread: 9/16-18 UNF, 18 TPI
- Major diameter: 0.5625 in (14.29 mm)
- Tap drill: 33/64 = 0.5156 in (13.10 mm)
- Thread engagement: 65 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 18 × 0.0469 / 0.01299 = 65 %.

## FAQ

**What size drill for a 9/16-18 tap?**
Use a 33/64 drill (0.5156 in, 13.10 mm). It leaves about 65 % thread.

**Can I use a metric drill for 9/16-18?**
Yes. A 13.1 mm drill gives about 65 % thread.

**How tight should a 9/16 grade 5 bolt be?**
About 121 ft-lb dry for 9/16-18 UNF.

Source page: https://machinistcharts.com/tap-drill/9-16-18/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)