I measure out at 28" around on a short, petite, well-endowed frame.  Add 5" and that's 33", right?  The 34's I've been wearing are a little snug now since I've gained a couple lbs, so I decided to go to a 36, which is what I got.  A 36DD.  The cups are about a half cup too large, but that's fine.  The band is TIGHT on me on the largest setting.  Other than that, it's a comfortable bra.  I might try to get another one in a 38D and see how that fits.  But do be aware, unless I got an odd one, the bands are small.