5/16-18 UNC tap drill size

F drill — 0.2570 in / 6.53 mm (77% thread)

Major Ø 0.3125 in · 18 threads per inch · coarse series. The chart pick targets 75% thread; nearby standard drills:

Drill	Decimal (in)	mm	Thread %
H	0.2660	6.76	64%
17/64	0.2656	6.75	65%
G	0.2610	6.63	71%
F	0.2570	6.53	77%

Cut taps only — roll-form taps need a larger hole. 65–75% suits general work; drop toward 55% in tough material. Verify before you cut.

Same size, fine series: 5/16-24 UNF