5/16 inch in mm and decimal

5/16 in = 0.3125 in = 7.938 mm. Closest metric drill: 7.9 mm.

Taps that use a 5/16 drill

Drill	Type	Inch	mm	Difference
L	letter	.2900 in	7.366	-22.5 thou
M	letter	.2950 in	7.493	-17.5 thou
19/64	fraction	.2969 in	7.541	-15.6 thou
N	letter	.3020 in	7.671	-10.5 thou
5/16	fraction	.3125 in	7.938	0.0 thou
O	letter	.3160 in	8.026	3.5 thou
P	letter	.3230 in	8.204	10.5 thou
21/64	fraction	.3281 in	8.334	15.6 thou
Q	letter	.3320 in	8.433	19.5 thou

1 thou = 0.001 in = 0.0254 mm. Drills a few thou apart are usually interchangeable for clearance holes, but not for tap drills or reamer pilots.