Quick answer
For a 5/16-18 UNC tap, use an F drill bit. The decimal equivalent is about 0.257 inches, or roughly 6.53 mm.
Thread
5/16-18 UNC
Tap drill
F drill bit
Decimal size
0.257 in
Close metric size
6.53 mm
Why this uses a letter bit
Many tap drill charts use number and letter drill sizes because they land closer to ideal thread percentages than common fractional bits.
Material notes
For steel, use cutting oil and steady pressure. For aluminum, keep chips cleared so they do not gall and damage the new threads.
Useful tools for this job
A letter drill index pairs well with tap work because sizes like F, I, and Q show up often on inch-thread charts.
Frequently asked questions
Can I use 1/4 inch for a 5/16-18 tap?
1/4 inch is smaller than the standard F drill and can make tapping harder. Use F when available.
Is 5/16-18 coarse thread?
Yes. 5/16-18 is the common UNC coarse-thread size.