I am a 34A and a medium fits perfectly.
I want to start out by saying that I would 100% recommend this bra in spite of the (small) issue I had with it. This bra is incredibly comfortable and supportive, and I just love how much coverage it offers (I've never liked high-necked bras because the feeling of fabric up to my collarbone felt odd to me, but this one is incredible.) The only issue I have is with the optional padding that comes with the bra. I got this bra in white, and while it is lined heavily enough so as not to be see-through, it is pretty apparent when I'm cold.... So I like to wear it with the removable padding when worn without a shirt. However, while I love BP's bra padding (it NEVER rolls up in the wash!!!!!) it is thicker than most removable pads and textured. The pads show up like two bumpy circles standing off my chest in this bra, where it doesn't do that in my other BP bra. Not terrible, but not great. I'm sure if I were more well-endowed this might not be an issue, but it is worth noting. The pads also move toward the center of the bra since there is no defined cup between the liner and the top layer of the bra. You can probably sub out some removable pads from a different bra to solve one or both issues. I just deal with the BP cups because it's not a big enough issue to merit removing them with how comfortable the bra is!