# 1/4-28 tap drill size

The tap drill for a 1/4-28 UNF thread is a #3 drill (0.2130 in, 5.41 mm). It leaves about 80 % thread. The closest metric drill is 5.4 mm, which leaves 81 % thread.

## Drill options for 1/4-28 UNF

| Drill | Inch | Thread % |
|---|---|---|
| #3 | 0.2130 | 80 % |
| 5.3 mm | 0.2087 | 89 % |
| 5.4 mm | 0.2126 | 81 % |
| 5.5 mm | 0.2165 | 72 % |

## 1/4-28 thread data

- Thread: 1/4-28 UNF, 28 TPI
- Major diameter: 0.2500 in (6.35 mm)
- Tap drill: #3 = 0.2130 in (5.41 mm)
- Thread engagement: 80 %

## Method

Thread % = TPI × (D − drill) / 0.01299 = 28 × 0.0370 / 0.01299 = 80 %.

## FAQ

**What size drill for a 1/4-28 tap?**
Use a #3 drill (0.2130 in, 5.41 mm). It leaves about 80 % thread.

**Can I use a metric drill for 1/4-28?**
Yes. A 5.4 mm drill gives about 81 % thread.

**How tight should a 1/4 grade 5 bolt be?**
About 9.7 ft-lb dry for 1/4-28 UNF.

Source page: https://machinistcharts.com/tap-drill/1-4-28/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)