Place the measuring tape across your back at band level, under each arm, and around the fullest part of your bust. Make sure the tape is level and isn't too tight by taking a deep breath in and out, then record your measurement.

Find your most accurate measurement by wearing a non-padded bra. Place the measuring tape around your torso directly under your bust, where your bra band sits. Make sure it is pulled tight but is not uncomfortable and that the tape is level then take your measurement. 

Find your bra size using your underbust and bust measurements in the size finder.