# #5-40 tap drill size

The tap drill for a #5-40 UNC thread is a #38 drill (0.1015 in, 2.58 mm). It leaves about 72 % thread. The closest metric drill is 2.6 mm, which leaves 70 % thread.

## Drill options for #5-40 UNC

| Drill | Inch | Thread % |
|---|---|---|
| #38 | 0.1015 | 72 % |
| 2.5 mm | 0.0984 | 82 % |
| 2.6 mm | 0.1024 | 70 % |
| 2.7 mm | 0.1063 | 58 % |

## #5-40 thread data

- Thread: #5-40 UNC, 40 TPI
- Major diameter: 0.1250 in (3.17 mm)
- Tap drill: #38 = 0.1015 in (2.58 mm)
- Thread engagement: 72 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 40 × 0.0235 / 0.01299 = 72 %.

## FAQ

**What size drill for a #5-40 tap?**
Use a #38 drill (0.1015 in, 2.58 mm). It leaves about 72 % thread.

**Can I use a metric drill for #5-40?**
Yes. A 2.6 mm drill gives about 70 % thread.

Source page: https://machinistcharts.com/tap-drill/5-40/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)