# Letter X drill size

A letter X drill is 0.3970 in (10.08 mm). It sits between letter W and letter Y in the letter series; the nearest metric drill is 10.1 mm.

## Letter X drill data

- Series: Letter drill
- Decimal: 0.3970 in
- Metric: 10.08 mm (10.0838 mm exact)
- Nearest metric drill: 10.1 mm (+0.016 mm)
- Nearest fractional: 25/64" (0.3906 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 10.0 mm | 0.3937 | −0.084 mm |
| 10.1 mm | 0.3976 | +0.016 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| #1 | Number | 0.2280 | 5.79 |
| Letter W | Letter | 0.3860 | 9.80 |
| 25/64" | Fractional | 0.3906 | 9.92 |
| Letter X | Letter | 0.3970 | 10.08 |
| Letter Y | Letter | 0.4040 | 10.26 |
| 13/32" | Fractional | 0.4062 | 10.32 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| M12 × 1.75 | 10.2 mm | 84 |
| 7/16-20 UNF | 25/64 | 62 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| 3/8" bolt | 0.3750" | 0.0220" | 0.0110" | – |
| M10 bolt | 10 mm | 0.08 mm | 0.04 mm | Tighter than close fit (10.5 mm) |


## FAQ

**What size is a letter X drill?**
A letter X drill is 0.3970 in, which is 10.08 mm (10.0838 mm exactly).

**What is the metric equivalent of a letter X drill?**
The nearest metric drill is 10.1 mm, 0.016 mm larger. The other close size is 10.0 mm.

**What thread can I tap with a letter X drill?**
It leaves about 84 % thread in M12 × 1.75 and 62 % in 7/16-20 UNF.

**Which fractional drill is closest to letter X?**
25/64 in (0.3906 in) is the closest, 0.0064 in smaller.

**What does a letter X drill clear?**
It clears a M10 bolt with 0.08 mm total clearance (0.04 mm per side); for metric bolts: tighter than close fit.

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