You don’t need a PhD in lingerie. Just a soft measuring tape and a friend who won’t laugh. Step one: measure your underbust (right below your boobs). That’s your band size.
Step two: measure the fullest part of your bust. Don’t squish yourself. Let it be natural. Then subtract your band measurement from your bust measurement. Every inch equals one cup size.
One inch? You’re an A. Two inches? B. Three? C. Four? D. Five? DD (or E). Six? You’re in the big leagues (F/G). See? Simple math with a saucy payoff.