10-32 Tap Drill Size
Unified fine (UNF) · 32 threads per inch · ASME B1.1, class 2B
What size drill bit for a 10-32 tap?
To scale section of the tapped hole
Drill options % of thread
| Drill | Inch | mm | Thread | 2B |
|---|---|---|---|---|
| 5/32" | 0.1563 | 3.97 | ✓ | |
| #22 | 0.1570 | 3.99 | ✓ | |
| 4 mm | 0.1575 | 4.00 | ✓ | |
| #21Use | 0.1590 | 4.04 | ✓ | |
| #20 | 0.1610 | 4.09 | ✓ | |
| 4.1 mm | 0.1614 | 4.10 | ✓ | |
| 4.2 mm | 0.1654 | 4.20 | ✗ | |
| #19 | 0.1660 | 4.22 | ✗ |
By material Aluminum, brass, mild steel: #21 (76%) · Stainless, tool steel: #20 (71%)
75% (orange tick) is the usual target. Hard or gummy metals like stainless are often tapped at 60 to 65% to spare the tap; aluminum, brass and plastic take 75% or a little more. The last column checks the hole against the 2B minor diameter limits (0.156 in to 0.164 in).
10-32 UNF ASME B1.1, class 2B
| Major diameter | 0.1900 in / 4.83 mm |
|---|---|
| Threads per inch | 32 TPI (pitch 0.0313 in) |
| Pitch diameter (basic) | 0.1697 in |
| Minor diameter 2B | 0.1562–0.1641 in |
| Thread-forming tap drill | #16 0.1770 in |
Clearance hole ASME B18.2.8
For a screw that passes through the part. Normal fit is the default; close fit locates the part more accurately but leaves no room for hole position error.
Hole depth rule of thumb
- 0.190 inthread length for full strength in steel (1 × D)
- 0.285 inin cast iron or brass (1.5 × D), up to 0.380 in in aluminum
- +0.156 inextra drill depth below the thread for a plug tap (5 pitches), +0.063 in with a bottoming tap
Shop notes
The #21 drill (0.1590 in) is the standard for #10-32 and gives about 76% thread. Unlike its coarse sibling, #10-32 has very little room on either side: the fine pitch means each thousandth of drill size moves the thread percentage by about two and a half points.
That is why the 5/32 that people grab instead is a poor swap. At 0.1562 in it cuts about 83% thread, legal for the 2B minor diameter but noticeably harder to tap, and in steel that extra torque is often what breaks a #10 tap. If you have no #21, a #20 (71%) or a 4.1 mm metric drill (70%) is the safer side to miss on.
You meet #10-32 most in pneumatic fittings, where the small ports are tapped 10-32, and on instruments and fixtures where the finer pitch adjusts more precisely and holds better in thin walls.
10-24 or 10-32?
Both are #10, both are 0.190 in across, and the screws look identical until you count threads: 24 per inch against 32. A 10-24 screw will start in a 10-32 hole and lock up within a turn. If a #10 screw will not go, a thread gauge or a known good nut settles it in seconds.
Questions
What size is a #21 drill bit?
A #21 drill is 0.1590 in (4.04 mm), between 5/32 (0.1563 in) and 11/64 (0.1719 in). It is the standard tap drill for #10-32.