An Expert’s Three-Point Fit Guide
One of the top bra fit experts in the industry says you can tell if a bra fits properly by checking three things.
The gore, the portion of the band between the cups, should fit flat against your chest wall between your breasts. If the gore is not flat against your chest, the bra is the wrong size - usually with a band size too large and a cup size too small. Try a smaller band and a larger cup size until you get your center gore to fit flat against your chest.
Underwires should completely surround your breasts and lay against your chest wall. If the underwire is a finger's width or more lower than your beast, the band size is probably too small. If the underwire lies against your breast tissue, the cups are probably too small. Adjust band size and cup size until you get the underwire in the proper position.
The band should lie flat against your back in a straight line below your shoulder blades; it should not pull-up toward your shoulders. A band that cuts into your skin or leaves red marks is probably too small. A band that "rides up" on your back is probably too large. Remember if you go down a band size you'll need to go up a cup size and vice versa.