Check for the Right Fit as of 2012-11-22 14:45:25

Really, to know if your bra fits, you should be checking more than just your cups and band but most of it can be fixed if you have those right ;) .

Determine if the band fits

The band of your bra should be responsible for holding most of the weight of your breasts.

It should be snug but not too tight. You should be able to accommodate one or two fingers under the band.
Check that the entire band is parallel to the floor. It should be at the same height on the front and on the back.

Signs that you're wearing the wrong band size

If the bra rides up on the back, then it is too big and is not supporting your boobs properly, now the bra straps are and this is not what you want.
Your breasts are popping out under the cups.
The straps keep sliding off of your shoulders.

It's a good idea to try on your new bras always on the loosest hook in order to be able to switch to the tighter hooks when your band stretches out.

Determine if the cup fits

All these should be true:

The centerpiece of the cups should lay flat on your sternum.
Each of your breasts should be enclosed naturally inside each cup.
The underwires should not be poking into breast tissue on the outer sides. Look here for how to find out where your breast tissue ends.
Check that there is one continuous curve across the top of your breasts through the cup to the bottom of the underwire so no breast tissue is bulging out. The cups aren't empty and wrinkled. There's no orange in a glass effect.

Find a better fitting bra

You can determine a good size for you by checking if your bra shows signs of bad fit.

This means that for example if the band is riding up, you should try a smaller band size. Keep in mind that cup sizes are related to the band size, so if you are wearing a 32D that fits well in the cups, but you want to try a bra in a smaller band, try a 30DD.

If your cups are too small, try bigger cups. If your cups are too big, try smaller ones.

If your bra is wrong on every end and simple one-step adjustments won't help, maybe try out a calculator, for a first idea of where to start looking, or even better, use the fitting help available from the Bratabase community if you upload pictures of yourself wearing a bra.

Always try to move all of your breast tissue into the cups! The cups are meant to accommodate all of your bust and not just some of it.