# 1/4-20 tap drill size

The tap drill for a 1/4-20 UNC thread is a #7 drill (0.2010 in, 5.11 mm). It leaves about 75 % thread. The closest metric drill is 5.1 mm, which leaves 76 % thread.

## Drill options for 1/4-20 UNC

| Drill | Inch | Thread % |
|---|---|---|
| #7 | 0.2010 | 75 % |
| 5.0 mm | 0.1968 | 82 % |
| 5.1 mm | 0.2008 | 76 % |
| 5.2 mm | 0.2047 | 70 % |

## 1/4-20 thread data

- Thread: 1/4-20 UNC, 20 TPI
- Major diameter: 0.2500 in (6.35 mm)
- Tap drill: #7 = 0.2010 in (5.11 mm)
- Thread engagement: 75 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 20 × 0.0490 / 0.01299 = 75 %.

## FAQ

**What size drill for a 1/4-20 tap?**
Use a #7 drill (0.2010 in, 5.11 mm). It leaves about 75 % thread.

**Can I use a metric drill for 1/4-20?**
Yes. A 5.1 mm drill gives about 76 % thread.

**How tight should a 1/4 grade 5 bolt be?**
About 8.5 ft-lb dry for 1/4-20 UNC.

Source page: https://machinistcharts.com/tap-drill/1-4-20/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)