There is an actual science/formula for measuring for bra size.
The down side is, with a kajillion outfits making bras, your best bet is to go TRY THEM ON!
I really and truly, went to a professional, sells bras only store and got measured, by a supposed, professionally trained bra fitter, last fall, they told me I was 34D. ROFLMAO! The store did not have a single bra in a 34D that could hold a candle to my girls.
A 34 is to tight, and a D, well, if I take a deep breath, it would be a peep show!
I am somewhere in between a 36 and a 38 band size, and a D or DD cup, depends on the bra.
I am very PICKY about my bras. They HAVE to be underwire, comfy, hold the girls up and be at least a little bit cute/sexy. The last time I bought bras, I shopped and tried on bras for 3 months before I was happy.
And I spent the money, but it's an item you wear every day, and it needs to work for whatever works for you.