# Letter I drill size

A letter I drill is 0.2720 in (6.91 mm). It is the standard tap drill for 5/16-24 UNF; the nearest metric drill is 6.9 mm.

## Letter I drill data

- Series: Letter drill
- Decimal: 0.2720 in
- Metric: 6.91 mm (6.9088 mm exact)
- Nearest metric drill: 6.9 mm (−0.009 mm)
- Nearest fractional: 17/64" (0.2656 in)
- Standard tap drill for: 5/16-24 UNF

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 6.9 mm | 0.2717 | −0.009 mm |
| 7.0 mm | 0.2756 | +0.091 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| 17/64" | Fractional | 0.2656 | 6.75 |
| Letter H | Letter | 0.2660 | 6.76 |
| Letter I | Letter | 0.2720 | 6.91 |
| Letter J | Letter | 0.2770 | 7.04 |
| 9/32" | Fractional | 0.2812 | 7.14 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| 5/16-24 UNF | I | 75 |
| M8 × 1.25 | 6.8 mm | 67 |
| M8 × 1 | 7 mm | 84 |
| 5/16-18 UNC | F | 56 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| #12 screw | 0.2160" | 0.0560" | 0.0280" | – |
| M6 bolt | 6 mm | 0.91 mm | 0.45 mm | Normal fit (normal 6.6, loose 7 mm) |
| 1/4" bolt | 0.2500" | 0.0220" | 0.0110" | – |


## FAQ

**What size is a letter I drill?**
A letter I drill is 0.2720 in, which is 6.91 mm (6.9088 mm exactly).

**What is the metric equivalent of a letter I drill?**
The nearest metric drill is 6.9 mm, 0.009 mm smaller. The other close size is 7.0 mm.

**What tap uses a letter I drill?**
A letter I drill is the standard tap drill for 5/16-24 UNF (about 75 % thread).

**Which fractional drill is closest to letter I?**
17/64 in (0.2656 in) is the closest, 0.0064 in smaller.

**What does a letter I drill clear?**
It clears a 1/4" bolt with 0.0220" total clearance (0.0110" per side).

Source page: https://machinistcharts.com/drill-size/letter-i/
Updated: 2026-10-06
Publisher: MachinistCharts (https://machinistcharts.com)