When brands use double letters, it usually means that they’re using UK sizing. (Ewa Michalak does use UK cup sizes, even though they use EU band sizes.) UK sizing can be a bit confusing, but for the most part every size after D has a double letter (except for E for some reason):
AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K, KK, L, LL, M
Each double-letter size is its own full cup size, not a half size. I can see why you’re confused, though, because a little while ago there was a department store brand that made half-sizes called BB (almost B) and CC (almost C). This is not the case with UK sizing—here, each double letter is its own size. 🙂
Read: Sports Bra Guide
Another source of confusion is that US sizing tends to run large in the cup after D. Some retailers even equate a US D cup to a UK DD cup, since US sizes after D seem to take larger “steps” after that. (I can confirm this somewhat—I’ve tried on “30DDDD” Chantelle bras that fit like a UK 30FF, even though theoretically they’re equivalent to a UK 30F.)
A quick note about AAA and AA cups: I’ve heard from some people that AAA and AA cups are “flatter”/shallower versions of A cups, and I’ve heard from others that they are simply smaller all around. Either way, I think it’s pretty safe to say that they are whole cup sizes, because the volume between them seems to be a whole size. (What even is a “full cup size” anyway? It seems so subjective; I sometimes wonder why this question is so common.)
But, as a general rule, double letters are not half sizes, as each size represents an inch more of difference between the under bust and over bust measurement.
