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What Are Bra Sister Sizes and Why Do They Work?

You may have heard lingerie fans criticizing the idea of “sister sizes” in bras as a general bad practice. That wearing a sister size is “wrong,” won't be a good fit, and is just a way for brands to sell you more product.

However, sister sizing isn’t a negative concept. At its core, it’s a neutral fact describing how bra sizes work.

So what is a sister size? How do you find yours? And when is it actually super beneficial to your lingerie shopping experience?

How does sister sizing work?

A “sister size” is a size that has the same cup volume, but a different band length, as another size.

So if a bra is a size 32B, its sister size “up” would be 34A, and its sister size “down” would be 30C. Despite having different numbers, these bras all hold the same amount of tissue in their cups, with different band lengths.

Bra Sister Sizes Chart

This chart from Triumph via Chatelaine shows the sister sizes for some common US core sizes. The sizes in the same color are all sister sizes to each other.

Why do sister sizes have a bad reputation?

Most people’s first experience with sister sizing is in a bra boutique that doesn’t carry or stock their size. They might hear, “We’re out of the 32C, but you can wear the 34B!” Or, “You measure at a 28D, but no one makes that size, so you are a 32B!”

In this case, the sister size is seen as being used to make a sale, rather than to benefit the bra-wearer. Boutiques and brands might encourage sister sizing to ensure the customer leaves with something, or to build loyalty to their company, instead of sending the customer somewhere they will actually find their size.

And say you're someone who wears a fuller cup size, but is sister sizing into a bra that isn't designed for a fuller cup size. For instance, you normally wear a 30FF in full bust brands but want to try a 36DD in a core-size fashion brand. In this case, the cups might technically fit, but you won't get the same amount of support from the band (because it's too big) or the cup (because it's designed for someone with less extreme proportions).

This makes sister sizing automatically feel like the wrong way to go. However, sister sizing isn’t always a bad thing.

When is sister sizing helpful?

  • When your bra band doesn’t fit. If the bra is riding up, you can sister size down to keep your cup volume the same but add more support through the band. If the bra is so small you can’t even fasten it but the cups are great, you can sister size up to feel more comfortable. In both cases, the cup volume will be the same.
  • When a bra is known to run small or large. Just like in clothing, bras do not have a standardized size system. Different brands use different measuring systems and size charts. Plus, there are often variations between styles for the same brand. If you’re shopping online and know a certain bra runs small in the band, you can order a sister size up to get a better fit.
  • When you love a bra but it doesn’t come in your preferred size. I like to wear a 32G, but I sister size to a 36E or DD  when I want to wear brands like Agent Provocateur and Chantal Thomass. I won’t get the same amount of support as I do in my preferred size, but I get to wear some really cute things that I otherwise would miss out on!!
  • When you’re buying a gift. If you’re absolutely set on a certain bra for your recipient but it isn’t made in their size, picking a sister size is a (slightly risky) way to give the gift you really want to give—especially if the bra is a novelty style that isn’t meant to be supportive.

There are so many shadows around the world of bra fit, but you don’t have to be a professional fit expert to get the basics of how bra sizing works. Understanding sister sizing helps you be a smarter consumer, ensuring your next lingerie shopping experience is as easy and fun as possible.

Quinne Myers

Quinne Myers is a lingerie expert living in Brooklyn, NY, where she creates quippy written content, crafts dreamy illustrations, and runs the ethically-made loungewear line, she and reverie.

8 Comments on this post

  1. Hilary says:

    Sister sizing doesn’t work when you’re a 28 band size…

  2. LadyOwl says:

    How odd that you do not have such standard there. In Europe we have EN 13402 dress-size standard, which also has rules for determining bra sizes as well. It makes European bra sizes much easier to understand (not talking about UK which seems to be at odds with sensible practices).

  3. AlexaFaie says:

    I wish this worked better in terms of how the volume is distributed. Because then I could get a smaller band size and wear an extender to get the right fit. But the cup widths don’t narrow enough on the smaller bands to make that work.
    I’m still waiting for a plunge with a wire width/cradle whatever you want to call it of a UK 32DD (the last size at which the wires sit perfectly at my IMF) but with the cup volume of a UK 32G. I can’t seem to find anything with enough projection or close together enough cups on my band size. And I’ve also discovered that my band size is only a 32 in Bravissimo Satine bras. Elsewhere, a 32 might just stretch to 30″ which makes it too tight for my rib cage shape. But if I go up a band, the cups become further apart.

    I bought 4 sister sized bras from Ewa Michalak hunting for enough projection. The smallest cup fit well to my IMF, but not enough space in it. The larger cups were too wide and tall and not projected enough. Even the largest cup volume which arguably should have been too big volume wise didn’t have enough volume where I needed it and caused me to spill out whilst there was ample space in the sides where my boobs don’t live.

    It’s getting to the point where I feel like I have to chop my boobs off entirely because there’s nothing that fits me right. Can’t even exercise them smaller as no sports bra has worked either. And regular bras aren’t supportive enough. It’s a shame because I love my boobs. But the lack of options I have makes me feel a lot of hate.

    It’s not my fault that my body is too far outside of what bra makers expect. I would order custom if I could find someone who made custom fit bras which are anything close to what I need. Honestly if anyone knows of someone who custom makes plunge bras in large sizes, please let me know!

    It’s not like I want much. Just soft fabrics (not lace or mesh) in a plain colour. Not even fussed about patterns. Solid colours are fine. With a small amount of “padding” in the form of just enough lining to make the cup have some structure. (I’ve tried unlined bras and they just don’t work for me, even when otherwise an identical pattern). And that’s it.

    I don’t want anything fancy. I just want to be comfortable! I am sick of wearing the best fit I can find and it being painful and having the staff have no idea what else to suggest. Thus is at Bravissimo too so somewhere specific to large cups!

  4. thefitter says:

    As a fitter I find sister sizing incredibly helpful when it comes to aiding customers in finding what bra fit gives them the most support and is the most comfortable! Great post!

  5. Sarah says:

    Sister sizing is super useful as someone who is technically a 39 band size but generally takes a 38DDD or a 40D/42C in M Frame bras. I don’t have much padding on my under bust area and I have had blisters due to the band rubbing against the bone. Especially with these back smoothing bras and sports brasI have to size up to 40DD(In a full cup)/40DDD(less coverage bras.) in a firm band.

  6. Jamel Harris says:

    My wife is a 44DDD what would be her sister sizes? The chart didn’t have her size :/

  7. Paige says:

    This is why i LOVE @Playful Promises. They have it right! 2 or 3 different processes for Small Medium and Large busted ladies. They re jig the style to work for larger sizes as it moves up the chart.

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