A #21 drill measures 0.1590 inch (4.039 mm). The closest fractional drill is 5/32" (0.1563") and the closest metric drill is 4.0 mm (0.1575"). It is the standard tap drill for #10-32.
What are the #21 drill equivalents?
| Drill | System | Decimal (in) | mm | Difference (in) |
|---|---|---|---|---|
| #21 | number | 0.1590 | 4.039 | 0 |
| 5/32" | fraction | 0.1563 | 3.969 | -0.0027 |
| #22 | number | 0.1570 | 3.988 | -0.0020 |
| 4.0 mm | metric | 0.1575 | 4.000 | -0.0015 |
| #20 | number | 0.1610 | 4.089 | +0.0020 |
| 4.1 mm | metric | 0.1614 | 4.100 | +0.0024 |
Tap drill for: #10-32.
Drill sets that include this size
Number and letter sizes come in indexed sets; buy the set rather than single drills.
What speed should a #21 drill run?
| Material | RPM (HSS) | Feed (in/rev) |
|---|---|---|
| 6061-T6 aluminum | 4,805 | 0.0032 |
| 360 free-machining brass | 3,604 | 0.0029 |
| 1018 cold rolled steel | 1,922 | 0.0024 |
| 304 stainless steel | 961 | 0.0016 |
Starting points from conservative HSS drilling speeds. Small drills often call for more RPM than a benchtop drill press offers; run at the top speed and feed gently. Use cutting oil in steel and stainless, and clamp the work: never hold it by hand.
Cutting fluid for drilling
Frequently asked questions
What is a #21 drill in inches?
0.1590 inch, which is 4.039 mm. Number drills get smaller as the number rises, so the next number up is a smaller drill. If you do not have this exact size, 4.0 mm is within 0.0015 inch.
What tap uses a #21 drill?
It is the standard 75% thread tap drill for #10-32. Tap drills are chosen to leave about 75% of full thread depth, the balance of strength and tapping torque that published charts use.