# Letter S drill size

A letter S drill is 0.3480 in (8.84 mm). It sits between letter R and letter T in the letter series; the nearest metric drill is 8.8 mm.

## Letter S drill data

- Series: Letter drill
- Decimal: 0.3480 in
- Metric: 8.84 mm (8.8392 mm exact)
- Nearest metric drill: 8.8 mm (−0.039 mm)
- Nearest fractional: 11/32" (0.3438 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 8.8 mm | 0.3465 | −0.039 mm |
| 8.9 mm | 0.3504 | +0.061 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| Letter R | Letter | 0.3390 | 8.61 |
| 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 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| M10 × 1.25 | 8.8 mm | 71 |
| M10 × 1.5 | 8.5 mm | 60 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| 5/16" bolt | 0.3125" | 0.0355" | 0.0177" | – |
| M8 bolt | 8 mm | 0.84 mm | 0.42 mm | Close fit (close 8.4, normal 9 mm) |


## FAQ

**What size is a letter S drill?**
A letter S drill is 0.3480 in, which is 8.84 mm (8.8392 mm exactly).

**What is the metric equivalent of a letter S drill?**
The nearest metric drill is 8.8 mm, 0.039 mm smaller. The other close size is 8.9 mm.

**What thread can I tap with a letter S drill?**
It leaves about 71 % thread in M10 × 1.25 and 60 % in M10 × 1.5.

**Which fractional drill is closest to letter S?**
11/32 in (0.3438 in) is the closest, 0.0042 in smaller.

**What does a letter S drill clear?**
It clears a M8 bolt with 0.84 mm total clearance (0.42 mm per side); for metric bolts: close fit.

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