# Letter G drill size

A letter G drill is 0.2610 in (6.63 mm). It sits between letter F and letter H in the letter series; the nearest metric drill is 6.6 mm.

## Letter G drill data

- Series: Letter drill
- Decimal: 0.2610 in
- Metric: 6.63 mm (6.6294 mm exact)
- Nearest metric drill: 6.6 mm (−0.029 mm)
- Nearest fractional: 17/64" (0.2656 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 6.6 mm | 0.2598 | −0.029 mm |
| 6.7 mm | 0.2638 | +0.071 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| 1/4" | Fractional | 0.2500 | 6.35 |
| Letter F | Letter | 0.2570 | 6.53 |
| Letter G | Letter | 0.2610 | 6.63 |
| 17/64" | Fractional | 0.2656 | 6.75 |
| Letter H | Letter | 0.2660 | 6.76 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| 5/16-18 UNC | F | 71 |
| M8 × 1.25 | 6.8 mm | 84 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| #12 screw | 0.2160" | 0.0450" | 0.0225" | – |
| M6 bolt | 6 mm | 0.63 mm | 0.31 mm | Normal fit (normal 6.6, loose 7 mm) |
| 1/4" bolt | 0.2500" | 0.0110" | 0.0055" | – |


## FAQ

**What size is a letter G drill?**
A letter G drill is 0.2610 in, which is 6.63 mm (6.6294 mm exactly).

**What is the metric equivalent of a letter G drill?**
The nearest metric drill is 6.6 mm, 0.029 mm smaller. The other close size is 6.7 mm.

**What thread can I tap with a letter G drill?**
It leaves about 71 % thread in 5/16-18 UNC and 84 % in M8 × 1.25.

**Which fractional drill is closest to letter G?**
17/64 in (0.2656 in) is the closest, 0.0046 in larger.

**What does a letter G drill clear?**
It clears a 1/4" bolt with 0.0110" total clearance (0.0055" per side).

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