# #20 drill size

A #20 drill is 0.1610 in (4.09 mm). It sits between #21 and #19 in the number series; the nearest metric drill is 4.1 mm.

## #20 drill data

- Series: Number drill
- Decimal: 0.1610 in
- Metric: 4.09 mm (4.0894 mm exact)
- Nearest metric drill: 4.1 mm (+0.011 mm)
- Nearest fractional: 5/32" (0.1562 in)

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 4.0 mm | 0.1575 | −0.089 mm |
| 4.1 mm | 0.1614 | +0.011 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| 5/32" | Fractional | 0.1562 | 3.97 |
| #21 | Number | 0.1590 | 4.04 |
| #20 | Number | 0.1610 | 4.09 |
| #19 | Number | 0.1660 | 4.22 |
| 11/64" | Fractional | 0.1719 | 4.37 |
| Letter A | Letter | 0.2340 | 5.94 |

## Threads this drill can tap

| Thread | Standard drill | Thread % with this drill |
|---|---|---|
| #10-32 UNF | #21 | 71 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| #3 screw | 0.0990" | 0.0620" | 0.0310" | – |
| #4 screw | 0.1120" | 0.0490" | 0.0245" | – |
| #5 screw | 0.1250" | 0.0360" | 0.0180" | – |
| M3.5 bolt | 3.5 mm | 0.59 mm | 0.29 mm | Normal fit (normal 3.9, loose 4.2 mm) |
| #6 screw | 0.1380" | 0.0230" | 0.0115" | – |
| M4 bolt | 4 mm | 0.09 mm | 0.04 mm | Tighter than close fit (4.3 mm) |


## FAQ

**What size is a #20 drill?**
A #20 drill is 0.1610 in, which is 4.09 mm (4.0894 mm exactly).

**What is the metric equivalent of a #20 drill?**
The nearest metric drill is 4.1 mm, 0.011 mm larger. The other close size is 4.0 mm.

**What thread can I tap with a #20 drill?**
It leaves about 71 % thread in #10-32 UNF.

**Which fractional drill is closest to #20?**
5/32 in (0.1562 in) is the closest, 0.0048 in smaller.

**What does a #20 drill clear?**
It clears a M4 bolt with 0.09 mm total clearance (0.04 mm per side); for metric bolts: tighter than close fit.

Source page: https://machinistcharts.com/drill-size/number-20/
Updated: 2026-10-06
Publisher: MachinistCharts (https://machinistcharts.com)