# #21 drill size

A #21 drill is 0.1590 in (4.04 mm). It is the standard tap drill for #10-32 UNF; the nearest metric drill is 4.0 mm.

## #21 drill data

- Series: Number drill
- Decimal: 0.1590 in
- Metric: 4.04 mm (4.0386 mm exact)
- Nearest metric drill: 4.0 mm (−0.039 mm)
- Nearest fractional: 5/32" (0.1562 in)
- Standard tap drill for: #10-32 UNF

## Nearest metric drills

| Metric drill | Inch | Difference |
|---|---|---|
| 4.0 mm | 0.1575 | −0.039 mm |
| 4.1 mm | 0.1614 | +0.061 mm |

## Nearby inch drills

| Drill | Series | Inch | mm |
|---|---|---|---|
| 5/32" | Fractional | 0.1562 | 3.97 |
| #22 | Number | 0.1570 | 3.99 |
| #21 | Number | 0.1590 | 4.04 |
| #20 | Number | 0.1610 | 4.09 |
| 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 | 76 |
| #10-24 UNC | #25 | 57 |

## Clearance

| Fastener | Diameter | Clearance | Per side | ISO 273 |
|---|---|---|---|---|
| #3 screw | 0.0990" | 0.0600" | 0.0300" | – |
| #4 screw | 0.1120" | 0.0470" | 0.0235" | – |
| #5 screw | 0.1250" | 0.0340" | 0.0170" | – |
| M3.5 bolt | 3.5 mm | 0.54 mm | 0.27 mm | Normal fit (normal 3.9, loose 4.2 mm) |
| #6 screw | 0.1380" | 0.0210" | 0.0105" | – |
| M4 bolt | 4 mm | 0.04 mm | 0.02 mm | Tighter than close fit (4.3 mm) |


## FAQ

**What size is a #21 drill?**
A #21 drill is 0.1590 in, which is 4.04 mm (4.0386 mm exactly).

**What is the metric equivalent of a #21 drill?**
The nearest metric drill is 4.0 mm, 0.039 mm smaller. The other close size is 4.1 mm.

**What tap uses a #21 drill?**
A #21 drill is the standard tap drill for #10-32 UNF (about 76 % thread).

**Which fractional drill is closest to #21?**
5/32 in (0.1562 in) is the closest, 0.0028 in smaller.

**What does a #21 drill clear?**
It clears a M4 bolt with 0.04 mm total clearance (0.02 mm per side); for metric bolts: tighter than close fit.

Source page: https://machinistcharts.com/drill-size/number-21/
Updated: 2026-10-06
Publisher: MachinistCharts (https://machinistcharts.com)