With your bra on, measure firmly around your rib cage, directly underneath your breasts. The tape measure should be horizontal around your body and should not drop in the back. This is your underbust or band measurement.
Wrap the measuring tape somewhat loosely around the fullest part of your chest (at nipple level). This is best done while you are wearing a bra (non-padded). Make sure the tape measure is straight across your back, and sits parallel to the floor all the way around your body. Round to the nearest whole number in inches