In theory, the number is inches around your ribcage right below your breasts and the letter is the difference between that and the measurement around the fullest part of your breasts, where each letter is about an inch. E.g. if you measure 32″ rib and then 34″ bust it’d be 32B, or if 32″ and 35″ then 32C.
In reality bodies are complicated and breasts come in all shapes and sizes and manufacturers don’t stick to the numbers anyway (just look at other clothes sizings and how much they vary) and there are many different kinds of bras. So good luck.
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