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Wear a thin, unlined bra. If you can, get someone else to measure you. If you measure yourself, stand in front of a mirror and practice perfect posture to make sure the tape measure is parallel to the floor.
Run a tape measure around your ribcage right under your bust.
Place the tape measure under your arms around the fullest part of your chest.
Your recommended bra size is a starting point that should be used as a guide. Try some on so you can see how the bra feels on your body. Every bra should fit snug—wearing it too loose won't provide you with the support you need.
Bra support comes mostly from the band so it's important that it fits quite close to your ribcage. It can take some work (but, it's worth it!) so be patient and adjust your bra size up or down until you nail it.