Some confusion about sister sizes

Since I’m getting a fair amount of traffic the past couple days (thank you Reddit for not getting angry at me for posting my own blog, and thank you By Baby’s Rules for linking here on Facebook!), I thought it would be a good time to ask a question that my math/engineering type brain has been struggling with lately:

Why do sister sizes work?

For a while, I was still wearing 30 bands, and while trying on those sizes, I found that a 30H fit me best, but that the band was too big. So I ordered the sister size, 28HH, and when I got it, it was too small in the cup. I was complaining about this to my sister when she pointed out that really, mathematically, sister sizing doesn’t really make sense.

It’s basic bra knowledge, if you go down a band size, go up a cup size to make up for it. But wait… Does that really make logical sense? I know that manufacturers use that chart with all the wire widths and the wires used to make a 30H really ARE the same wires used to make a 28HH… but does this make sense?



For example, lets say that you have a woman with a 29 inch underbust and a 40 inch bust. She decides she feels most comfortable in a 30 band, which would put her in a 30GG (once she calculated her cup size from 40 – 30 = 10).

But, let’s say our 29 inch underbust decides she’d rather use a 28 band size from the start! So now, her cup size is 40 – 28 = 12, and her cup size is HH… so now she has a 30GG or a 28HH to choose from, based on measurements? But what about the whole sister sizes thing?

I understand that bra sizing is an art, not a science, and that many things can affect the way your bra fits etc. etc. but I’m just trying to talk straight math here. A woman who wears a 28H has a bust that’s a full inch smaller than a woman who wears a 30GG, so why are they sister sizes?

Basically, once you find a cup that fits, shouldn’t the corresponding cup size actually be down a band and upĀ two cup sizes? Can someone enlighten me please? Does anyone else find that they fit best in sizes that aren’t technically sister sizes (such as me, with a 30H and a 28J, but 28HH is usually a tad small)?