Flat bust is 3 to 4 cm.

Let's take a look at popular science first: upper bust: the fullness of the chest is one week, which is the bust size of women.

Lower bust: refers to the bust at the base of the breast.

Chest grade: 1, and the upper bust-lower bust of standard A cup is equal to 7.5~ 10cm. 2, the chest is smaller than the upper bust-the lower bust is equal to 6~7cm. 3, the chest is small, the upper bust-the lower bust is equal to 5cm. 4, flat chest upper bust-lower bust is equal to 3~4cm. 5, flat upper bust-lower bust is equal to 2.5cm.