I hope this helps :
all these years of CDing, i’ve been wearing the wrong size of bra. I based my first bra purchase off of my swimsuit chest size, 38. That was highly inaccurate, and nowhere close.
I too was confused about bra band size and cup size, until the kind folks at the breast form store helped me out. The breastforms I wanted to buy were out of stock, so I picked another one, but then they asked me if I knew the difference about concavity.
From there, they got me to measure my over/under bust measurements. That was when I found out my bra band size was 48. Nowhere near the 38 I thought I was.
Because I had already spent a sizeable chunk of $$$ on the new breastforms, I had to make due with an existing 44/46 bra band bra, using a bra extender. I have to say it fit really well and much more comfortable.