# #31 drill size

A #31 drill is 0.1200 in (3.05 mm). It sits between #32 and #30 in the number series; the nearest metric drill is 3.0 mm.

## #31 drill data

- Series: Number drill
- Decimal: 0.1200 in
- Metric: 3.05 mm (3.0480 mm exact)
- Nearest metric drill: 3.0 mm (−0.048 mm)
- Nearest fractional: 1/8" (0.1250 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 3.0 mm | 0.1181 | −0.048 mm |
| 3.1 mm | 0.1220 | +0.052 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| 7/64" | Fractional | 0.1094 | 2.78 |
| #32 | Number | 0.1160 | 2.95 |
| #31 | Number | 0.1200 | 3.05 |
| 1/8" | Fractional | 0.1250 | 3.17 |
| #30 | Number | 0.1285 | 3.26 |
| Letter A | Letter | 0.2340 | 5.94 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| M3.5 × 0.6 | 2.9 mm | 58 |
| #6-40 UNF | #33 | 55 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| #1 screw | 0.0730" | 0.0470" | 0.0235" | – |
| #2 screw | 0.0860" | 0.0340" | 0.0170" | – |
| M2.5 bolt | 2.5 mm | 0.55 mm | 0.27 mm | Normal fit (normal 2.9, loose 3.1 mm) |
| #3 screw | 0.0990" | 0.0210" | 0.0105" | – |
| #4 screw | 0.1120" | 0.0080" | 0.0040" | – |
| M3 bolt | 3 mm | 0.05 mm | 0.02 mm | Tighter than close fit (3.2 mm) |


## FAQ

**What size is a #31 drill?**
A #31 drill is 0.1200 in, which is 3.05 mm (3.0480 mm exactly).

**What is the metric equivalent of a #31 drill?**
The nearest metric drill is 3.0 mm, 0.048 mm smaller. The other close size is 3.1 mm.

**What thread can I tap with a #31 drill?**
It leaves about 58 % thread in M3.5 × 0.6 and 55 % in #6-40 UNF.

**Which fractional drill is closest to #31?**
1/8 in (0.1250 in) is the closest, 0.0050 in larger.

**What does a #31 drill clear?**
It clears a M3 bolt with 0.05 mm total clearance (0.02 mm per side); for metric bolts: tighter than close fit.

Source page: https://machinistcharts.com/drill-size/number-31/
Updated: 2026-10-06
Publisher: MachinistCharts (https://machinistcharts.com)