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 up 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)?

14 thoughts on “Some confusion about sister sizes

  1. I’ve done the math thing several times. And it doesn’t add up, no matter what sizing method you use. It makes me want to bang my head on the wall.

    I definitely think part of it is the shaping of the bra. Maybe the cup in the “sister size” is more shallow with extra width for example. I’ve read a few posts about this, but for the life of me I can’t remember what blogger did them. The only one I can find is from My Curves.

    • Yeah I think that is definitely a problem! For example, I bought the Curvy Kate Emily in a 28HH after near success with the Princess in a 30H, and its cups end up getting spread across my chest (aka more shallow and wide). A while after that, I finally went in for a professional fitting and tried on the Romance in a 30H (which is basically the same as the Emily cut wise) and it fit perfectly in the cups (though the band was much too large).

      Someone commented on my Emily post and said the band was too small and the cups were too big, and that’s why it wasn’t fitting well. It’s true that the 30H cups were a better fit, but I measure 28.5″ underbust and a 30 band is very large! It really leaves me in quite the sizing dilemma. I have an Ewa Michalak on the way from Big Bra Bar though, so hopefully her narrow wires can be a solution for me. I think a lot of small band large cup women have the problem with cups ending up too wide.

      …that got kind of off topic, but whatever.

      • Off topic, but totally applies to me. I have the same issues with super wide wires. I have to order up several cupsizes in order to get the depth that I need. I have the CK Ella (28J), Angel (28JJ), and Princess (28JJ) that all fit in the depth, but the wires are all almost to my back! I also have a few Ewa bras on order through BBB. She actually contacted me today about it. She has a Comexim bra in 28J that looks gorgeous, but I’m unsure of the sizing. It sounds like we may have some similar fitting issues with the CK bras.

        Have you started using You put in your measurements, the measurements of your bras (fitting or not) to help find bras that may fit. You can browse other bras to look for specific measurements, buy preowned bras, sell bras that don’t fit, etc. It was a great help when I was trying to find a cup depth that would work in CK bras for me.

  2. I’m just guessing here, but I think it’s partly the problem of odd-number sizes – you need to calculate based on raw data, then adjust it to a band size. In your example the woman is settling for either the sister size of 30GG or the sister size of 28HH – she’s really a 29H, which doesn’t exist. If she measured 28 underbust, then 28H and 30GG would be right.

    I’ve also read somewhere (I can’t remember where, I’m really sorry) that some companies use the double letters as sort of half-sizes, so sisters aren’t necessarily, depending on brand.

    also check out Braless in Brasil’s post 🙂

    • Well, if we changed the example to a woman with a 30 inch inderbust and 40 inch bust (true 30GG), but she’s trying a brand that has a loose band, it would still be suggested that she try a 28H, assuming the cups are correct in the GG. But, the cups in a 28H are designed for a 39 inch bust so she’s still going to run into the same problem 😦 And if all bras were made perfectly, with no loose bands, then the concept of sister sizes wouldn’t even matter really.

      And you know, I read someone say that on Reddit… but I’ve never personally seen a brand that does that, and I’ve tried most of them (Curvy Kate, Fantasie, Alegro, Freya, Panache (and Cleo and Masquerade), etc.). I think more important than half sizes in cups would be half sizes in bands! That would make this “sister sizes don’t make mathematical sense” argument nonexistent… because then they would make sense! 28HH=29H=30GG… because all of those women have a 40″ bust!

  3. I remember seeing a blog post that tried to prove that mathematically sister sizes were the same size, but I don’t remember what blog it was. I think it might have been this one:

    When I put 29″ and 38″ into the Sophisticated Pair calculator, I get 28GG on tightest fit and 30G on the moderate fit. So, their calculator makes it seem like sister sizes work. But it all varies depending on the bra. Soooo, complicated.

    • Eh, that doesn’t really prove sister sizes to me as it proves why a 36D is larger than a 32D. I don’t know if I’ll ever really feel clear on this sister size business, especially since so far it hasn’t really worked on me very well, so I don’t have any personal evidence for it.

      Yeah very complicated indeed :/

    • Glad to see I’m not the only one out there confused by this! Have you found that the down a band, up 2 cups system works better for you too? Like I said in the post, I’m a 28J pretty often, but very rarely am I a 30HH (I usually fit better in a 30H).

  4. I actually seem to have half a cup size difference when I go up or down a band size … very odd. It obviously depends on the person/breast shape and/or density

  5. I’ve always thought about that! It doesn’t add up! I thought of an explanation a few days ago while I was far from sober and I cannot recall what it was and I really wish I did! PS, I am also studying engineering (mechanical) and obsessed with breasts and about the same bra size as you and have the same bra preferences as you. . .when I found your blog a few weeks ago I got really excited and new this would be one of my new favorite blogs 🙂

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