Why do so many women wear the wrong bra size?

Bra size isn't just letters. AA, A, B, C... are the cup size (aka breast size), there is also the number for your "band size", the underbust measurement.

How can I tell if my bra fits correctly?

Things need to check :

• BAND: when hooked around your body, you should be able to snake the width of your finger underneath.  In other words, it should be tight, but not so tight it creates bulge around the band of the bra

• CUP: looking in the mirror, your boob should fit in the cup without squeezing out the top (which will create a "double boob" in a t-shirt) or leaving extra material (which will also look funny in a t-shirt)

• STRAPS: shoulder straps should again be tight without digging into your skin, and not loose enough to fall down your shoulders.