Here is a link: www.reddit.com/r/ABraThatFits/ Read it all. It's magnificent.
1)So you strip down naked, bend at the waist, parallel to the ground, and measure around your back and hanging breasts loosely without squishing or reshaping them. Write that number down.
2)Measure your ribcage right under your breasts where the band should fit. Write that number down.
Now punch the numbers into this excellent size calculator: www.sophisticatedpair.com
3) Stare agog at the size bra you SHOULD be wearing. I will tell you I have been wearing a 36 A or a heavily padded B. Sometimes a 38 band if I can find it (you can't) because I thought I had a big rib cage and something wasn't fitting. This I knew. Well, problem solved. Apparently I should be in a...
Yeah, I remeasured a few times just to be sure. I am skeptical. And now the hunt is on for a store that actually carries such a rando size to actually try on (a Target purchase this ain't). And so I say that maybe the reason we're all wearing the wrong bra size is because most non-bra-exclusive stores only carry about 5 sizes that we all shoehorn ourselves into and make due.