Measure around the fullest part of your breasts, over your nipple area. While holding both ends of the tape with one hand at the side of your breast, use your free hand to press the measuring tape in between your breasts to your chest. This will provide a more accurate bust size.

Measure your chest below your breasts - make sure the tape is around your back and under your armpits. Check in the mirror to ensure the measuring tape is parallel to the floor along your back. The tape should be snug around your body without any fingers underneath the tape.

Subtract the rib cage measurement from the bust size measurement. The difference represents your letter cup size. （Around 1 in.=A, 2 in.=B, 3 in.=C, 4 in.=D, 5 in.=E, 6 in=F）