# 5/8-18 tap drill size

The tap drill for a 5/8-18 UNF thread is a 37/64 drill (0.5781 in, 14.68 mm). It leaves about 65 % thread. The closest metric drill is 14.7 mm, which leaves 64 % thread.

## Drill options for 5/8-18 UNF

| Drill | Inch | Thread % |
|---|---|---|
| 37/64 | 0.5781 | 65 % |
| 14.6 mm | 0.5748 | 70 % |
| 14.7 mm | 0.5787 | 64 % |
| 14.8 mm | 0.5827 | 59 % |

## 5/8-18 thread data

- Thread: 5/8-18 UNF, 18 TPI
- Major diameter: 0.6250 in (15.88 mm)
- Tap drill: 37/64 = 0.5781 in (14.68 mm)
- Thread engagement: 65 %

## Method

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

## FAQ

**What size drill for a 5/8-18 tap?**
Use a 37/64 drill (0.5781 in, 14.68 mm). It leaves about 65 % thread.

**Can I use a metric drill for 5/8-18?**
Yes. A 14.7 mm drill gives about 64 % thread.

**How tight should a 5/8 grade 5 bolt be?**
About 170 ft-lb dry for 5/8-18 UNF.

Source page: https://machinistcharts.com/tap-drill/5-8-18/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)