# Letter O drill size

A letter O drill is 0.3160 in (8.03 mm). It sits between letter N and letter P in the letter series; the nearest metric drill is 8.0 mm.

## Letter O drill data

- Series: Letter drill
- Decimal: 0.3160 in
- Metric: 8.03 mm (8.0264 mm exact)
- Nearest metric drill: 8.0 mm (−0.026 mm)
- Nearest fractional: 5/16" (0.3125 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 8.0 mm | 0.3150 | −0.026 mm |
| 8.1 mm | 0.3189 | +0.074 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| Letter N | Letter | 0.3020 | 7.67 |
| 5/16" | Fractional | 0.3125 | 7.94 |
| Letter O | Letter | 0.3160 | 8.03 |
| Letter P | Letter | 0.3230 | 8.20 |
| 21/64" | Fractional | 0.3281 | 8.33 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| 3/8-16 UNC | 5/16 | 73 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| 5/16" bolt | 0.3125" | 0.0035" | 0.0018" | – |
| M8 bolt | 8 mm | 0.03 mm | 0.01 mm | Tighter than close fit (8.4 mm) |


## FAQ

**What size is a letter O drill?**
A letter O drill is 0.3160 in, which is 8.03 mm (8.0264 mm exactly).

**What is the metric equivalent of a letter O drill?**
The nearest metric drill is 8.0 mm, 0.026 mm smaller. The other close size is 8.1 mm.

**What thread can I tap with a letter O drill?**
It leaves about 73 % thread in 3/8-16 UNC.

**Which fractional drill is closest to letter O?**
5/16 in (0.3125 in) is the closest, 0.0035 in smaller.

**What does a letter O drill clear?**
It clears a M8 bolt with 0.03 mm total clearance (0.01 mm per side); for metric bolts: tighter than close fit.

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