#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 at a glance
Decimal, metric and the nearest sizes in the other series.
- Decimal inch
- 0.1590"
- Millimetres
- 4.04 mm 4.0386
- Nearest metric drill
- 4.0 mm −0.039 mm
- Nearest fractional
- 5/32" 0.1562", −0.0028"
- Hole area
- 0.0199 in² 12.81 mm²
- Tap drill for
- #10-32 UNF
Nearest metric drills
Metric drills step by 0.05 mm below 3 mm and by 0.1 mm from 3 mm. Difference is metric minus inch size.
| Metric drill | Inch | Difference | In inches | Hole |
|---|---|---|---|---|
| 4.0 mm | 0.1575" | −0.039 mm | −0.0015" | Smaller |
| 4.1 mm | 0.1614" | +0.061 mm | +0.0024" | Larger |
Nearby inch drills
The next size down and up in each inch series, so you can pick a substitute from the drawer.
Threads a #21 drill can tap
Threads from the tap tables on this site that this drill can tap, with the thread percentage it leaves (55–85 % shown).
| Thread | Standard drill | Thread with this drill | Suits |
|---|---|---|---|
| #10-32 UNF | #21 | 76 % | Standard drill |
| #10-24 UNC | #25 | 57 % | Hard, tough metals |
Clearance for screws and bolts
Bolts and screws a #21 hole clears, with the diametral clearance and the gap each side. Metric sizes are compared with ISO 273.
| Fastener | Diameter | Clearance | Per side | ISO 273, metric only |
|---|---|---|---|---|
| #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) |
Inch clearance is the drill size minus the basic major diameter. A drilled hole usually cuts slightly oversize.
Number drill index
Every size in the series, holes drawn to scale.
Common questions
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.