---
type: "drill"
designation: "F"
series: "letter"
series_name: "Letter Drill Sizes"
diameter_in: 0.257
diameter_mm: 6.5278
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/letter/f
---

# F Drill Bit Size — 0.2570 in / 6.528 mm

A F drill bit is 0.2570 in (6.528 mm) in diameter. It belongs to the letter series of standard twist drills. It is the recommended tap drill for 5/16"-18 UNC.

- **Designation**: F
- **Series**: Letter
- **Diameter**: 0.2570 in (6.528 mm)

## F Drill Diameter

| Property | Value |
| --- | --- |
| Designation | F |
| Series | Letter |
| Diameter (in) | 0.2570 |
| Diameter (mm) | 6.528 |

## F Drill Equivalents in the Other Size Series

The nearest standard drill to F in each of the other three series. Nothing here is an exact match unless the difference column reads 0.0000 in.

| Series | Nearest drill | Diameter (in) | Diameter (mm) | Difference |
| --- | --- | --- | --- | --- |
| Number | [#1](https://shopmath.org/drills/number/1.md) | 0.2280 | 5.791 | -0.0290 in |
| Fractional | [1/4](https://shopmath.org/drills/fractional/1-4.md) | 0.2500 | 6.350 | -0.0070 in |
| Metric | [6.5 mm](https://shopmath.org/drills/metric/6-5mm.md) | 0.2559 | 6.500 | -0.0011 in |

## Threads Tapped With a F Drill

| Thread | Pitch | Thread engagement (%) |
| --- | --- | --- |
| [5/16"-18 UNC](https://shopmath.org/threads/unc/5-16-18.md) | 18 TPI | 76.9 |

_Engagement is what the real drill leaves with a cut tap, not the requested target._

## Sources and Method for the F Drill Size

### Drill index

- **Source**: Number/letter: bd_warehouse drill_sizes.csv (Apache-2.0) cross-checked against Wikipedia Drill bit sizes (CC BY-SA 4.0). Fractional: computed 1/64 in steps per ANSI B94.11M. Metric: computed BS 328 increment pattern.
- **Method**: compiled
- **Verified against**: Wikipedia Drill bit sizes (CC BY-SA 4.0); 31/31 checks passed

### Assumptions

- Drill diameters are nominal. A worn or run-out drill cuts oversize; measure the hole, not the shank, when the fit matters.
- Dataset scope: Number sizes #61-#80 are stored but tagged demandTier 'extended' — query-space-map.md Appendix A found the tail thinning in the 60s. Sizes below #80 and metric sizes above 25.0 mm excluded.

**Data last verified**: August 1, 2026

## Common Questions About the F Drill Bit

### What size is a F drill bit size?

A F drill bit is 0.2570 in (6.528 mm) in diameter.

### What is the metric equivalent of a F drill bit?

The closest metric drill to F (0.2570 in (6.528 mm)) is 6.5 mm at 0.2559 in (6.500 mm) — a difference of 0.0011 in.

### What fraction is a F drill bit?

F is 0.2570 in (6.528 mm), which is not itself a fractional size. The closest fractional drill is 1/4 at 0.2500 in (6.350 mm).

### What thread does a F drill bit tap?

A F drill is the recommended tap drill for 5/16"-18 UNC. That leaves the thread engagement shown in the table above with a cut tap.

## Related pages

- Next size down (any series): [6.5 mm](https://shopmath.org/drills/metric/6-5mm.md) — 0.2559 in (6.500 mm)
- Next size up (any series): [6.6 mm](https://shopmath.org/drills/metric/6-6mm.md) — 0.2598 in (6.600 mm)
- Previous in the letter series: [E](https://shopmath.org/drills/letter/e.md)
- Next in the letter series: [G](https://shopmath.org/drills/letter/g.md)
- All letter sizes: [Letter drill sizes](https://shopmath.org/drills/letter.md)
- All drill sizes: [Drill bit size reference](https://shopmath.org/drills.md)
- Complete index by diameter: [Drill size chart](https://shopmath.org/charts/drill-sizes.md)
- Tap drills by thread: [Tap drill chart](https://shopmath.org/charts/tap-drill.md)

---

*Source: [ShopMath](https://shopmath.org/drills/letter/f) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*