The measurement is done by measuring just below a girl's bust and then measuring around the largest part of the bust size where the nipples are. Subtract the two and each inch in size is a cup size in letters like A, B, C and so on. How do you tell just by looking at the boobs? A cup looks like lemons, B cup like the size of oranges, C cup like grapefruit, D cup like cantaloupe. If the boobs bounce when the girl walks around they are definitely C cup or higher.