According to the barenecessities site, after DD, it depends on whether it is European sizing or UK sizing. So, on the one hand, a US DD = E = European E = UK DD; on the other hand, a US DDD = F = European F = UK E. A US DDDD = G = European G = UK F, and so on.
And even with this kind of chart, there is still no universal standard sizing. From brand to brand and style to style, bras will (obviously) fit differently. I also find that sports bras seem to fit small and most women seem to need to go up a size or so.