Quick answer
For an #8-32 UNC tap, use a #29 drill bit. The decimal equivalent is about 0.136 inches, or roughly 3.45 mm.
Thread
#8-32 UNC
Tap drill
#29 drill bit
Decimal size
0.136 in
Close metric size
3.45 mm
Where this size appears
#8-32 screws are common in small machine brackets, enclosures, electrical boxes, and light-duty metal projects.
How to avoid breaking the tap
Keep the tap square to the hole, use cutting fluid for metal, and back the tap out regularly to break chips. Small taps are easy to snap if forced.
Useful tools for this job
A #29 bit and #8-32 plug tap are a good exact-match purchase. A small tap set is better if you expect to thread multiple screw sizes.
Frequently asked questions
Is #8-32 the same as M4?
No. #8-32 is an inch thread and M4 is metric. They are close in diameter but not interchangeable.
Can I drill by eye for an #8-32 tap?
No. Use the correct numbered bit because small thread sizes are less forgiving than larger holes.