Wearing the correct bra cup size is important for comfort and appearance. An ill-fitting bra can pinch and chafe the skin, and it can also cause clothing to fit oddly. It's simple to measure yourself for the correct bra cup size at home. All you need is a tape measure.
Wrap the tape measure around the top part of the chest, going underneath the armpits but staying above the breast tissue. The resulting number is the band size. If the measurement is an odd number, round down to the next even number.
Wrap the tape measure around the fullest part of the bust, circling it completely around the body and across the nipples. Keep the tape measure mildly loose, as a too-tight measurement will result in a too-small bra cup.
Subtract the band measurement from the bust measurement. The resulting number is the cup size, with each inch of difference representing the next size up. For example, if the band measurement is 34 and the bust measurement is 36, the resulting cup size is a B. If the band measurement is 34 and the bust measurement is 37, the resulting cup size is a C.
If the difference between the bust measurement and the band measurement is less than 1 inch, the cup size needed will be either a AA or AAA. Try on both sizes to see which is more comfortable.