How to tell if your bra fits
1) It should fit snugly around. You ought to be able to fit one or two fingers under the band comfortably. If you are pulling down the back strap or pulling up on the shoulder straps throughout the day, then the band size (the # part of your bra size) is too big.
a) Can you fit 1 or 2 fingers comfortably? If yes, then the band is right size.
b) If you’re having a hard time putting one finger under the band, then move the hooks to the loosest fitting eyes. If it is still tight on the loosest fitting, then try on one in the next up band size.
c) If the band can easily be pulled away from your body, or if the band rises up your back, then go down one in the band size.
2) If you are wearing an underwire bra, the wire that comes up between the breasts should lay flat (or as close as possible) to your breast bone. If you are wearing a bra with no underwire, you should have two separate breasts, not just a “one big boob”.
3) If the bra is a soft cup bra, you shouldn’t have any extra, gapping fabric. If the bra is a molded cup you shouldn’t have any extra room in the cup. Your breast should completely fill the cup. If it is gapping and the band feels fine, then go down in the cup size.
4) You don’t want to be spilling out of the cup – not from the top, bottom or sides. If the band feels fine then go up a cup size.
5)If you raise your arms up (do this a few times), the bra should stay against your body, not lifting up or off. If this happens try a few things;
a) adjust the shoulder straps by loosing them
b) Pay attention to the band. Is it too tight? Or too loose? Or just right?
c) If the band is just right, and adjusting the shoulder straps didn’t work, then go up a cup size.
6) If you have a small frame or are petite, and you find a bra that fits the band, and you don’t quite fill the cup and you have already tried the next cup size down and it’s still not quite right – OR if the you have the “right” size, the band fits and you fill the cup, but the wire is poking your underarms, then try a Demi bra. A Demi style bra is different from a full coverage bra. The Demi has less wire, by about an inch or more.
7) Try on different styles of bras. Every style fits and fills different and can “shape” your breast differently.
8) If your bra fits correctly, it should not be painful or uncomfortable to wear.
Sources from 007b