For most of my life I believed that my bra size was the second. Then I took a course from bra fitter and I found out that I was wrong: the bounty series habitually used in Italy (1st, 2nd, 3rd and so on) actually refers only to the circumference of the chest, while the actual breast size is expressed in cups, using the letters of the alphabet from A (for smaller sizes) to K (in the larger sizes).
It becomes therefore essential to choose the right sports bra, measure much underbust band that the cups themselves, and above all understand what this refers to each measure. It would be good to take example from Anglo-Saxon women, already accustomed to determine their bra size considering both measures: bra sizes manufactured in the United States and in England, in fact, differ by combining the sizes of cups and the thorax, and are therefore more true to our forms and equipped with better fit than those produced in Italy.
In this link you can calculate the size of your bra by inserting the underbust measures and bust: you will get the perfect bra size, which holds without compressing and secured by the forms naturally.