# #6-40 tap drill size

The tap drill for a #6-40 UNF thread is a #33 drill (0.1130 in, 2.87 mm). It leaves about 77 % thread. The closest metric drill is 2.9 mm, which leaves 73 % thread.

## Drill options for #6-40 UNF

| Drill | Inch | Thread % |
|---|---|---|
| #33 | 0.1130 | 77 % |
| 2.8 mm | 0.1102 | 85 % |
| 2.9 mm | 0.1142 | 73 % |
| 3.0 mm | 0.1181 | 61 % |

## #6-40 thread data

- Thread: #6-40 UNF, 40 TPI
- Major diameter: 0.1380 in (3.51 mm)
- Tap drill: #33 = 0.1130 in (2.87 mm)
- Thread engagement: 77 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 40 × 0.0250 / 0.01299 = 77 %.

## FAQ

**What size drill for a #6-40 tap?**
Use a #33 drill (0.1130 in, 2.87 mm). It leaves about 77 % thread.

**Can I use a metric drill for #6-40?**
Yes. A 2.9 mm drill gives about 73 % thread.

Source page: https://machinistcharts.com/tap-drill/6-40/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)