Quick answer
For tapping threads, use the tap drill size for the exact thread pitch. Common UNC examples include 3.50 mm for #8-32, 4.00 mm for #10-24, 5.35 mm for 1/4-20, and 6.80 mm for 5/16-18.
#8-32 UNC
3.50 mm
#10-24 UNC
4.00 mm
1/4-20 UNC
5.35 mm
5/16-18 UNC
6.80 mm
3/8-16 UNC
7.94 mm
1/2-13 UNC
10.72 mm
Common UNC tap drill sizes
#8-32 UNC: #29 drill bit, 3.50 mm. Common machine screw thread for small brackets and electronics. #10-24 UNC: #25 drill bit, 4.00 mm. Coarse-thread option often used in general metal fastening. 1/4-20 UNC: #7 drill bit, 5.35 mm. Very common shop and fixture thread. 5/16-18 UNC: F drill bit, 6.80 mm. Common for medium-duty brackets and plates. 3/8-16 UNC: 5/16 in drill bit, 7.94 mm. Common for heavier brackets, fixtures, and shop projects. 1/2-13 UNC: 27/64 in drill bit, 10.72 mm. Common for larger fixtures and structural hardware.
Common #8 confusion
If you are drilling for an #8-32 machine screw tap, use a #29 drill bit. If you are drilling a pilot hole for a #8 wood screw, use the wood screw pilot-hole chart instead.
Why pitch matters
A 1/4-20 and 1/4-28 fastener have the same major diameter but different thread pitch, so they do not use the same tap drill size.
Useful tools for this job
Tap-and-die sets, individual taps, cobalt bits, and cutting fluid are useful when you are preparing a clean threaded hole.
Frequently asked questions
Can I round to the nearest drill bit?
Avoid casual rounding for tapped holes. If you do not own the exact size, use a tap drill chart to choose the nearest accepted equivalent for the thread and material.
Do metric and UNC taps use the same chart?
No. Metric taps are based on metric pitch and UNC taps are based on threads per inch, so use the chart for the thread system printed on the tap.