# Letter F drill size

A letter F drill is 0.2570 in (6.53 mm). It is the standard tap drill for 5/16-18 UNC; the nearest metric drill is 6.5 mm.

## Letter F drill data

- Series: Letter drill
- Decimal: 0.2570 in
- Metric: 6.53 mm (6.5278 mm exact)
- Nearest metric drill: 6.5 mm (−0.028 mm)
- Nearest fractional: 1/4" (0.2500 in)
- Standard tap drill for: 5/16-18 UNC

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 6.5 mm | 0.2559 | −0.028 mm |
| 6.6 mm | 0.2598 | +0.072 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| 1/4" | Fractional | 0.2500 | 6.35 |
| Letter E | Letter | 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 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| 5/16-18 UNC | F | 77 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| #12 screw | 0.2160" | 0.0410" | 0.0205" | – |
| M6 bolt | 6 mm | 0.53 mm | 0.26 mm | Close fit (close 6.4, normal 6.6 mm) |
| 1/4" bolt | 0.2500" | 0.0070" | 0.0035" | – |


## FAQ

**What size is a letter F drill?**
A letter F drill is 0.2570 in, which is 6.53 mm (6.5278 mm exactly).

**What is the metric equivalent of a letter F drill?**
The nearest metric drill is 6.5 mm, 0.028 mm smaller. The other close size is 6.6 mm.

**What tap uses a letter F drill?**
A letter F drill is the standard tap drill for 5/16-18 UNC (about 77 % thread).

**Which fractional drill is closest to letter F?**
1/4 in (0.2500 in) is the closest, 0.0070 in smaller.

**What does a letter F drill clear?**
It clears a 1/4" bolt with 0.0070" total clearance (0.0035" per side).

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