---
type: "drill"
designation: "R"
series: "letter"
series_name: "Letter Drill Sizes"
diameter_in: 0.339
diameter_mm: 8.6106
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/letter/r
---

# R Drill Bit Size — 0.3390 in / 8.611 mm

A R drill bit is 0.3390 in (8.611 mm) in diameter. It belongs to the letter series of standard twist drills.

- **Designation**: R
- **Series**: Letter
- **Diameter**: 0.3390 in (8.611 mm)

## R Drill Diameter

| Property | Value |
| --- | --- |
| Designation | R |
| Series | Letter |
| Diameter (in) | 0.3390 |
| Diameter (mm) | 8.611 |

## R Drill Equivalents in the Other Size Series

The nearest standard drill to R 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 | -0.1110 in |
| Fractional | [11/32](https://shopmath.org/drills/fractional/11-32.md) | 0.3438 | 8.731 | +0.0047 in |
| Metric | [8.6 mm](https://shopmath.org/drills/metric/8-6mm.md) | 0.3386 | 8.600 | -0.0004 in |

## Sources and Method for the R 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 R Drill Bit

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

A R drill bit is 0.3390 in (8.611 mm) in diameter.

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

The closest metric drill to R (0.3390 in (8.611 mm)) is 8.6 mm at 0.3386 in (8.600 mm) — a difference of 0.0004 in.

### What fraction is a R drill bit?

R is 0.3390 in (8.611 mm), which is not itself a fractional size. The closest fractional drill is 11/32 at 0.3438 in (8.731 mm).

## Related pages

- Next size down (any series): [8.6 mm](https://shopmath.org/drills/metric/8-6mm.md) — 0.3386 in (8.600 mm)
- Next size up (any series): [8.7 mm](https://shopmath.org/drills/metric/8-7mm.md) — 0.3425 in (8.700 mm)
- Previous in the letter series: [Q](https://shopmath.org/drills/letter/q.md)
- Next in the letter series: [S](https://shopmath.org/drills/letter/s.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/r) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*