# #4-40 tap drill size

The tap drill for a #4-40 UNC thread is a #43 drill (0.0890 in, 2.26 mm). It leaves about 71 % thread. The closest metric drill is 2.3 mm, which leaves 66 % thread.

## Drill options for #4-40 UNC

| Drill | Inch | Thread % |
|---|---|---|
| #43 | 0.0890 | 71 % |
| 2.2 mm | 0.0866 | 78 % |
| 2.3 mm | 0.0906 | 66 % |
| 2.4 mm | 0.0945 | 54 % |

## #4-40 thread data

- Thread: #4-40 UNC, 40 TPI
- Major diameter: 0.1120 in (2.84 mm)
- Tap drill: #43 = 0.0890 in (2.26 mm)
- Thread engagement: 71 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 40 × 0.0230 / 0.01299 = 71 %.

## FAQ

**What size drill for a #4-40 tap?**
Use a #43 drill (0.0890 in, 2.26 mm). It leaves about 71 % thread.

**Can I use a metric drill for #4-40?**
Yes. A 2.3 mm drill gives about 66 % thread.

Source page: https://machinistcharts.com/tap-drill/4-40/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)