Tap Drill Size Chart

#21 Drill Size

number drill

#21 0.159 in 4.04 mm

A #21 drill is the standard tap drill for #10-32.

What a #21 drill taps

Every thread this drill cuts between 50% and 85% of thread, deepest first. Outside that band it is the wrong drill for the job, so nothing below that line is listed.

Threads a #21 drill taps, with the percent of thread each one gets and the published chart drill for comparison.
Thread % thread Chart drill Chart %
#10-32 chart drill 76.4% #21 76.4%
#10-24 57.3% #25 74.8%

Drills either side

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#21 drill: common questions

What size is a #21 drill?
0.159 inches, which is 4.04 mm. It is a number drill.
What can I tap with a #21 drill?
2 threads: #10-32 at 76.4%, #10-24 at 57.3%.
Is a #21 drill the standard tap drill for anything?
Yes, for #10-32 (76.4% thread). That is the size published tap and drill charts give.
What drill is next to a #21?
Down: 4 mm at 0.1575 in. Up: #20 at 0.161 in.