# Letter H drill size

A letter H drill is 0.2660 in (6.76 mm). It sits between letter G and letter I in the letter series; the nearest metric drill is 6.8 mm.

## Letter H drill data

- Series: Letter drill
- Decimal: 0.2660 in
- Metric: 6.76 mm (6.7564 mm exact)
- Nearest metric drill: 6.8 mm (+0.044 mm)
- Nearest fractional: 17/64" (0.2656 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 6.7 mm | 0.2638 | −0.056 mm |
| 6.8 mm | 0.2677 | +0.044 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| Letter G | Letter | 0.2610 | 6.63 |
| 17/64" | Fractional | 0.2656 | 6.75 |
| Letter H | Letter | 0.2660 | 6.76 |
| Letter I | Letter | 0.2720 | 6.91 |
| 9/32" | Fractional | 0.2812 | 7.14 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| M8 × 1.25 | 6.8 mm | 77 |
| 5/16-18 UNC | F | 64 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| #12 screw | 0.2160" | 0.0500" | 0.0250" | – |
| M6 bolt | 6 mm | 0.76 mm | 0.38 mm | Normal fit (normal 6.6, loose 7 mm) |
| 1/4" bolt | 0.2500" | 0.0160" | 0.0080" | – |


## FAQ

**What size is a letter H drill?**
A letter H drill is 0.2660 in, which is 6.76 mm (6.7564 mm exactly).

**What is the metric equivalent of a letter H drill?**
The nearest metric drill is 6.8 mm, 0.044 mm larger. The other close size is 6.7 mm.

**What thread can I tap with a letter H drill?**
It leaves about 77 % thread in M8 × 1.25 and 64 % in 5/16-18 UNC.

**Which fractional drill is closest to letter H?**
17/64 in (0.2656 in) is the closest, 0.0004 in smaller.

**What does a letter H drill clear?**
It clears a 1/4" bolt with 0.0160" total clearance (0.0080" per side).

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