1.  Your undersbust is your chest measurement approximately 3″
below you nipples (where your bra band will sit) – with the tape measure
held TIGHT,  This is your underbust measurement NOT your bra size.
2.  Your overbust is the measurement around the fullest part of your bust boobs with a bra / breastforms on.

Find out what size breastform will fit the bra.
Select your bra size from the drop down menus,
identify which manufacturer’s breastforms you want the size for, and
then press ‘Calculate’.