At first I used a bra size calculator, with some success. I followed the instructions and it told me I need a 38 band size and at that time it was A cup, I also had a female friend help me..
As my breasts grew the cup size increased, I was never happy with how they fit.. so when I wore a 38C, I decided to go for a professional fitting at a lingerie shop. They were very professional.. they measured me and the lady said to me, try a 36D bra, so she helped me choose one. I choose a t-shirt style bra and I tried it on and wow... I had never worn such comfortable bra.
I bought a few bras that day in various styles and today I still wear a 36D.. That lady told me, if a bra fits right, it will almost feel like you not wearing one.