1. Measure your band size.
Run a tape around your body just undreneath your breast and note measurements in inches.your arms should be down. If this measurement is an odd number, you should try out bras in the size below your measurement and the size above(if you measure 31, try size 30 and 32). If it’s an even number ,that’s your size. A good test is to check that you can fit no more than a first under the back of bra.

2. Figure your cup size
The best way to do it is by usingthe bra you currently wear. The cup sizes are sized relative to the band,so if you were try a smaller band size, the cup would be too small.For every band size you decrease, increase the cups by one size(if your current size is 34D, and you measure 28 inches as band size, you should wear 28F).
Check around the cup, in front and sides, for bulging.
Make sure the underwires under your arms on the side are sitting on your ribs. If they cut into the sides of your breast, go for a larger cup size.

3.Take the mirror test.
Put on the chosen Bra, turn to one side and check if your bust stands half way between  your elbow and shoulder. Put on a R-shirt because it will display any bulges from cups that are too small.