ABTF calculator is awesomely good, but it has a flaw that it doesn't know how to compensate for thick backs and wide upper chest. It counts a whole lot as breast which isn't as it only reads measurements. I've noted how the error is getting less and less as I've grown bigger and lost muscle on my back. Lately the error has completely gone or its maybe half a cupsize tops.

But then again, my problem is breast shape, very wide and tall root translates to very large cupsize by measurement, but it doesn't easily matach typical cup depths which tend to be narrower and deeper than what would fit me the best. My current bra has that exact problem and its only getting closer now that I'm outgrowing it so its getting annoying at the same people.

People on forum keep repeating that ABTF doesn't work, it does and it does it really well, but it doesn't understand shapes as its nothing but sex digits counted into a ratio that is called cup size. So for wide chest and wide upper back, you have to compensate downwards, some times even two or three sizes to fit. Anyone with narrow back and projected breasts will get more realistic results. Read the FAQ, it explains the typical calculation errors too.