Quick answer
For a 1/4-20 UNC tap, use a #7 drill bit. The decimal equivalent is about 0.201 inches, or 5.35 mm.
Thread
1/4-20 UNC
Tap drill
#7 drill bit
Decimal size
0.201 in
Metric equivalent
5.35 mm
Why #7 is the standard size
A 1/4-20 tap cuts 20 threads per inch. The #7 drill leaves enough material for the tap to form usable threads without making the hole so tight that the tap binds.
When to adjust
Use cutting fluid, back the tap out to clear chips, and be more conservative in hard materials. If you are threading thin sheet metal or a specialty material, check the tap manufacturer chart for the exact thread percentage you want.
Useful tools for this job
A 1/4-20 tap, #7 cobalt drill bit, T-handle tap wrench, and cutting fluid are the natural bundle for this job.
Frequently asked questions
Can I use a 13/64 inch bit for a 1/4-20 tap?
13/64 inch is close, but #7 is the standard tap drill size. Use the standard size when you can, especially for metal.
Is 1/4-20 the same as 1/4-28?
No. 1/4-20 is coarse thread and 1/4-28 is fine thread. They use different tap drill sizes.