#21 Drill Bit Size and Diameter
A #21 drill bit measures 0.1590 in
4.04 mm · number-size drill · used as the 10-32 tap drill
The #21 is a numbered drill bit in the American gauge series, measuring 0.1590 inches (4.04 mm). It sits between a 5/32 in bit (0.1562 in) and a #20 bit (0.1610 in). It is best known as the standard tap drill for a 10-32 UNF fine thread.
At a glance
| Drill size | #21 |
|---|---|
| Diameter | 0.1590 in |
| Millimetres | 4.04 mm |
| Series | Number drill (gauge) 1–97 |
| Closest fractional | 5/32 in (0.1562 in, slightly smaller) |
What the #21 is used for
10-32 tap drill: drillØ = major − (pitch × 0.75) = 0.1900 − (0.0313 × 0.75) = 0.1666 in → nearest standard bit = #21 (0.1590 in)
A #21 hole before a 10-32 tap gives the standard ~75% thread engagement. Do not confuse it with the 10-24 coarse thread, which uses a #25 bit.
Related sizes
Other number drill bits
The neighbourhood of the #21 in the gauge series: