# #3-56 tap drill size

The tap drill for a #3-56 UNF thread is a #45 drill (0.0820 in, 2.08 mm). It leaves about 73 % thread. The closest metric drill is 2.1 mm, which leaves 70 % thread.

## Drill options for #3-56 UNF

| Drill | Inch | Thread % |
|---|---|---|
| #45 | 0.0820 | 73 % |
| 2.0 mm | 0.0787 | 87 % |
| 2.1 mm | 0.0827 | 70 % |
| 2.2 mm | 0.0866 | 53 % |

## #3-56 thread data

- Thread: #3-56 UNF, 56 TPI
- Major diameter: 0.0990 in (2.51 mm)
- Tap drill: #45 = 0.0820 in (2.08 mm)
- Thread engagement: 73 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 56 × 0.0170 / 0.01299 = 73 %.

## FAQ

**What size drill for a #3-56 tap?**
Use a #45 drill (0.0820 in, 2.08 mm). It leaves about 73 % thread.

**Can I use a metric drill for #3-56?**
Yes. A 2.1 mm drill gives about 70 % thread.

Source page: https://machinistcharts.com/tap-drill/3-56/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)