A while ago I posted about bra cup sizes and how I’d realized that I’d been wearing the wrong size bra. Since then I’ve settled on a bra size that’s a band size down and a cup size up from what I had worn for the past few years, and been pretty happy with results. At that point I was still trying to work out in my head why we use this relative size scheme where the cup sizes don’t mean anything by themselves – they’re relative to the band size, so a 36D and a 38D aren’t the same – in fact, a 38D and a 40C are the same, total volume wise.
I just ran across this informative blog post from Butterfly Collection that explained it in a way I could easily visualize. Basically all cups of the same letter are, in theory, the same depth but different widths. In other words, the depth – the amount your bust protrudes from your chest, e.g. as seen in profile – is the same in a 36D as a 38D, but the 38D is wider. This results in a larger total cup volume. The blog post has great illustrations of this.
This also reiterates why the standard sewing advice if you’re a C cup or larger is to use your upper bust measurement to choose your bodice size and do a full bust adjustment to accommodate your bust. Assuming your bodice size is correct for the rest of your measurements, you don’t need any more width all the way down your torso, you just need depth at the bust (which manifests itself in the pattern as added length, and a little both of added width, but only at the bust, not at the shoulders or the abdomen).
I’ve never been that spatially gifted, but sewing is definitely helping my 3D visualization skills!