R drill bit size

R drill = .3390 in = 8.61 mm. Closest metric drill: 8.6 mm.

Nearby drill sizes

Drill	Type	Inch	mm	Difference
O	letter	.3160 in	8.026	-23.0 thou
P	letter	.3230 in	8.204	-16.0 thou
21/64	fraction	.3281 in	8.334	-10.9 thou
Q	letter	.3320 in	8.433	-7.0 thou
R	letter	.3390 in	8.611	0.0 thou
11/32	fraction	.3438 in	8.731	4.7 thou
S	letter	.3480 in	8.839	9.0 thou
T	letter	.3580 in	9.093	19.0 thou
23/64	fraction	.3594 in	9.128	20.4 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.