I actually didn't know that..I always though cup size was the difference between the circumference of your chest right below you breasts, and the circumferance at your breast (the biggest part). If you have 1 inch difference, than you're a A cup, 2 inches, B cup, etc.... Am I getting it wrong? I'm confused!
I didn't know this until I was 30 years old-- I don't think most people do.
But guess what? Both things are true! The 1-inch, 2-inch, etc., difference rule is basically true, and cup size also varies based on band size. How can this be the case? Well, I wish my old fashion blog were still accessible (virus), because I had two huge posts on this. First, if you want to confirm that an A cup is not just a standard "A" volume pasted on to a smaller or larger band, all you have to do is look at a 32A and a 40A the next time you're bra shopping. Not just the band size, but the actual cup is MUCH larger (wish I had my comparison photos handy!)
So how does the 1" difference thing still make sense? I spent a while breaking it down on my blog so that it didn't seem so complicated (I used a cake and frosting example, LOL), but the short answer is-- geometry. We're not 1- or 2-dimensional, we're 3-dimensional. So an additional 1" in circumference/perimeter (1" longer perimeter around the bust than the underbust) translates into a larger volume (3 dimensions) when your underbust is larger. For a more extreme example-- imagine you have a golf ball and you wrap a tape measure around it. Then you add an extra 1" to the measured circumference. Imagine how many cotton balls would fill the gap you've created, between the golf ball and the tape measure. 1 or 2, maybe? Then take a beach ball and wrap a tape measure around IT and add an extra 1" to the measured circumference. Now imagine how many cotton balls you'd have to add to that gap to fill it-- more than 1 or 2, I can guarantee that! You could imagine extending a tape measure an extra 1" around the entire world, another sphere, and you would create a much bigger gap... YKWIM? I know it's hard to picture, but it really does pan out.
Now for an irresistible math interlude...
Going with my original cake example, if you had a Cake X that measured 8" on all sides (a weird, cube-like cake!), then the perimeter (as if you wrapped a tape measure around it one way) would be 32" (8+8+8+8). Now you frost the cake so that the sides are now 10" high, you have a perimeter of 36" (8+10+8+10). The difference in perimeters is 4".
Take another unfrosted Cake Y that is 9" on all sides, and the perimeter is 36" (9+9+9+9). Then add a layer of frosting so that it's 11" high on the sides, and the new perimeter is 40". The difference here in perimeters (unfrosted vs. frosted) is also 4".
Okay, so far, so good. Cake X is sorta like a 32D, and Cake Y is like a 36D, right?
But what's the difference in frosting volume?
For Cake X, we take the original volume of the unfrosted cake (8*8*8) and subtract it from the volume of the frosted cake (9*8*8-- because only the height of the cake changes-- the width and depth stay the same). That's 576-512, or 64 cubic inches of frosting, LOL.
For Cake Y, it's frosted (11*9*9) - unfrosted (9*9*9) = 891- 729 = 162 cubic inches of frosting.
That's a big difference in frosting.
Of course, we're not cubes, so this is not an exactly accurate representation, but you get the idea. It really is confusing, and stymied me for a while, too, but it makes sense when you do the math.
I do think that's one reason (knowing the 1" rule) people think that cup volume is a static thing. Well, that and the fact that American bra manufacturers in particular are terrible about making many sizes outside of 32-38 A-D (maybe DD)-- cheapskates! That encourages you to identify with a cup size-- KWIM? Like we all "know" that an A cup is small, a B-C is/looks average, a D is big, DD is huge and anything bigger is ginormous... But actually, since cup volume is based in part on band, a 30D or 32D doesn't look super big (I mean, not "flat," because it's on a petite person, but not "like a D cup" either). In fact, a 32D is approximately equivalent in actual volume to a 34C, a 36B or a 38A. Not so big. But it's hard to tell a woman who would be more comfortable in a 32D that she wears a D cup if she thinks of herself as a "B" cup type of gal. And it can induce tears to tell a woman she'd be much perkier and more comfortable in a G-cup! Ask me how I know!
But what happens is, if you don't know this (as I didn't!)... when a 34DD is too small in the cup, you move up to a 36DD because you "know" you "are" a DD cup and Victoria's Secret or whatever doesn't carry a DDD/E. Then, because a 36DD has a greater actual cup volume, it "fits"... But not really, because the band is too loose to provide great support. But you just keep on wearing a 36DD, because there "isn't" a 34DDD or whatever and you can't imagine "being" such a "huge" cup size. I wore a 38D for the longest time before a friend clued me in-- now I typically wear a 32FF (UK Size-- like a US 32DDDDD if it existed) when not pregnant, and I look much thinner and am waaaaay more comfortable.
I'll never forget, after having this same conversation with an older friend of mine (~65 years old), she bought new bras and was thrilled. Then we happened to be chatting with another 60+ year old friend of hers and my friend was extolling the virtues of her new bra size and sort of dropped a hint to her friend, like, oh, you probably wear a DD+, too. And her friend, was like, "No, you'd think so, but I'm a C!" Dude... This woman's boobs (in a bra) were in her lap. I'm sure she was wearing like a 40C and could have been wearing a 34DDD or more and been much happier. (Not that there's anything wrong with your boobs being in your lap, BTW, if you don't care or that's the look you're going for. But she might as well have not been wearing a bra at all.)