Quick answer
For a 3/8-16 UNC tap, use a 5/16 inch drill bit. The decimal equivalent is 0.3125 inches, or about 7.94 mm.
Thread
3/8-16 UNC
Tap drill
5/16 in drill bit
Decimal size
0.3125 in
Metric equivalent
7.94 mm
Why 5/16 inch works
The 5/16 inch hole leaves enough material for the 3/8-16 tap to cut strong coarse threads without excessive cutting load.
Project fit
3/8-16 threads are common on heavier brackets, fixtures, jigs, shop tables, and machinery hardware.
Useful tools for this job
A 3/8-16 tap and matching 5/16 inch bit are common enough to buy individually, but a fractional tap set may be better for repeated work.
Frequently asked questions
Can I use an 8 mm drill bit for a 3/8-16 tap?
8 mm is very close to 5/16 inch. For best results, use the charted 5/16 inch size when available.
Is 3/8-16 stronger than 5/16-18?
A 3/8 inch fastener is larger and often used for heavier loads, but strength depends on material, thread engagement, and installation.