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. If you mean a #8 wood screw pilot hole instead, use the #8 screw pilot-hole chart.
Thread
#8-32 UNC
Tap drill
#29 drill bit
Decimal size
0.136 in
Close metric size
3.45 mm
Different from
#8 wood screw pilot holes
#8-32 screw versus #8 wood screw
An #8-32 screw is a machine screw thread, so this page is for drilling a hole that will be tapped. A #8 wood screw uses a pilot hole in wood instead, usually 1/8 inch in hardwood or 7/64 inch in softwood.
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 the exact pair for this threaded hole. A small tap set is helpful if you expect to thread multiple screw sizes.
Frequently asked questions
What size drill bit for an #8-32 screw?
If you are tapping threads for an #8-32 screw, use a #29 drill bit. If the screw is only passing through a clearance hole, check the hardware or project chart for the clearance fit.
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.