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: 3.50 mm. Common machine screw thread for small brackets and electronics. #10-24 UNC: 4.00 mm. Coarse-thread option often used in general metal fastening. 1/4-20 UNC: 5.35 mm. Very common shop and fixture thread. 5/16-18 UNC: 6.80 mm. Common for medium-duty brackets and plates. 3/8-16 UNC: 7.94 mm. Common for heavier brackets, fixtures, and shop projects. 1/2-13 UNC: 10.72 mm. Common for larger fixtures and structural hardware.
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.