# Letter T drill size

A letter T drill is 0.3580 in (9.09 mm). It sits between letter S and letter U in the letter series; the nearest metric drill is 9.1 mm.

## Letter T drill data

- Series: Letter drill
- Decimal: 0.3580 in
- Metric: 9.09 mm (9.0932 mm exact)
- Nearest metric drill: 9.1 mm (+0.007 mm)
- Nearest fractional: 23/64" (0.3594 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 9.0 mm | 0.3543 | −0.093 mm |
| 9.1 mm | 0.3583 | +0.007 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| 11/32" | Fractional | 0.3438 | 8.73 |
| Letter S | Letter | 0.3480 | 8.84 |
| Letter T | Letter | 0.3580 | 9.09 |
| 23/64" | Fractional | 0.3594 | 9.13 |
| Letter U | Letter | 0.3680 | 9.35 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| M10 × 1 | 9 mm | 70 |
| M10 × 1.25 | 8.8 mm | 56 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| 5/16" bolt | 0.3125" | 0.0455" | 0.0227" | – |
| M8 bolt | 8 mm | 1.09 mm | 0.55 mm | Normal fit (normal 9, loose 10 mm) |


## FAQ

**What size is a letter T drill?**
A letter T drill is 0.3580 in, which is 9.09 mm (9.0932 mm exactly).

**What is the metric equivalent of a letter T drill?**
The nearest metric drill is 9.1 mm, 0.007 mm larger. The other close size is 9.0 mm.

**What thread can I tap with a letter T drill?**
It leaves about 70 % thread in M10 × 1 and 56 % in M10 × 1.25.

**Which fractional drill is closest to letter T?**
23/64 in (0.3594 in) is the closest, 0.0014 in larger.

**What does a letter T drill clear?**
It clears a M8 bolt with 1.09 mm total clearance (0.55 mm per side); for metric bolts: normal fit.

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