---
type: "drill"
designation: "T"
series: "letter"
series_name: "Letter Drill Sizes"
diameter_in: 0.358
diameter_mm: 9.0932
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/letter/t
---

# T Drill Bit Size — 0.3580 in / 9.093 mm

A T drill bit is 0.3580 in (9.093 mm) in diameter. It belongs to the letter series of standard twist drills.

- **Designation**: T
- **Series**: Letter
- **Diameter**: 0.3580 in (9.093 mm)

## T Drill Diameter

| Property | Value |
| --- | --- |
| Designation | T |
| Series | Letter |
| Diameter (in) | 0.3580 |
| Diameter (mm) | 9.093 |

## T Drill Equivalents in the Other Size Series

The nearest standard drill to T 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 | -0.1300 in |
| Fractional | [23/64](https://shopmath.org/drills/fractional/23-64.md) | 0.3594 | 9.128 | +0.0014 in |
| Metric | [9.1 mm](https://shopmath.org/drills/metric/9-1mm.md) | 0.3583 | 9.100 | +0.0003 in |

## Sources and Method for the T 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 T Drill Bit

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

A T drill bit is 0.3580 in (9.093 mm) in diameter.

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

The closest metric drill to T (0.3580 in (9.093 mm)) is 9.1 mm at 0.3583 in (9.100 mm) — a difference of 0.0003 in.

### What fraction is a T drill bit?

T is 0.3580 in (9.093 mm), which is not itself a fractional size. The closest fractional drill is 23/64 at 0.3594 in (9.128 mm).

## Related pages

- Next size down (any series): [9 mm](https://shopmath.org/drills/metric/9mm.md) — 0.3543 in (9.000 mm)
- Next size up (any series): [9.1 mm](https://shopmath.org/drills/metric/9-1mm.md) — 0.3583 in (9.100 mm)
- Previous in the letter series: [S](https://shopmath.org/drills/letter/s.md)
- Next in the letter series: [U](https://shopmath.org/drills/letter/u.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/t) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*