When taking measurements for these calculations, please use imperial units and wear a well-fitting bra.
Measure the circumference of your body a few inches above the fullest point of your bust. The measuring tape should pass under your arms, around the low point of your armpit.
Measure around the fullest part of your bust.
Measure from armpit height, over the peak of your breast, to just below your breast.
Measure around your torso about halfway between your underbust and your low waist (or around where your body naturally bends when you lean side to side). This may or may not be the narrowest part of your torso, which is okay.
Measure from underbust height to your low waist or where you would like your top to end.