Only knowing your cup size is not enough. You need to know the underbust size as well. A 32D and a 34C have cups with the same volume. Sure, there is still some variance but not as much as I thought before I learned that.
Edit: This calculator and the community of the same name on the-site-that-shall-not-be-named helped me a lot in finding my actual bra size. Now my only problem is that almost no company here has more than two or three bras in that size…
Yeah, the two measurements are really not enough to fit all the different boob shapes. And just offering different shapes with the same two measurements leads to problems for those who otherwise could rely on the two alone.
I have that problem with trousers where one measurement for width is not enough to fit both my waist and my hips. With bras it’s just that apparently you can’t have more than B or at maximum C if your underbust is 28/30. According to companies at least.