If you're not sure, always size up! Also remember that sometimes your butt size doesn't align with the rest of your clothing size. Obviously, know thine butt: if you have a big butt relative to the rest of your bod, get a size bigger than your regular clothing size! I usually wear an S for pants, but I wear a medium for these guys. 

 US size UK size Buttercup size
0 - 2 4 - 6 XS
4 8 S
6 10 M
8 12 L
10 14 XL