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The Thing About Sister Sizing Is…

what is sister sizing

The thing about sister sizing is… every bra maker and their mother has written about it. Amy of Cloth Habit, The Lingerie Addict and even Victoria Secret have had a word or two on the subject. Well, I’m joining the (cool) club and chiming in, mostly how you can use it to find your sister size if you don’t exactly fit into a particular band/cup size on the Madalynne X Simplicity 8229.

What is Sister Sizing?
One of my favorite bra making aphorisms is “fuzzy side up, picot edges inside.” It was even worthy of an Instagram post. Well, here’s one for sister sizing, “B is not a B is not a B.” You can thank Amy for that one. Another: “Cups are relative to band.” That came from Amber.

Sister size refers to different bra sizes that have the same cup volume even though the band size and cup letter are different. An example is a 34A and a 32B – they have the same cup volume, but a different band size. As the band size gets larger, cups will have the same volume only if the letter gets smaller. An easy way to think of it is that their reciprocals. As one moves up, the other moves down and vice versa.

Sister Size Down
This means one size smaller for the band and one letter bigger for the cups. Example: if you wear a 34C, your sister size down would be 32D.

Sister Size Up
This means one size larger for the band and one letter smaller for the cups. Using the same example as above, if you wear a 34C, your sister size up would be 36B.

How to Use Sister Sizing
The chart below represents all the sizes that are available for the 8229. If you can’t find your size on the chart, there may be a chance that there is a sister size you can use. One example that I used this past weekend during a 1-on-1 session using a 42DD pattern for a woman that measured a 44F. 

sister sizing chart

If You Use Your Sister Size, Do You Need To Make Changes To The Pattern?
Yes. The cup volume is the same, but the band size is different. When you take your sister size, you’ll have to increase or decrease the band, but this is a very easy adjustment. Amy demonstrates it here.

Sister Sizing Isn’t Always Correct:
You can’t always use your sister size. As Amber demonstrates in her post, sister size patterns aren’t always the same. In her post, she compares two sister sizes and explains that cups/frame can have different slopes/angles to allow for a shift in the distribution in weight while maintaining the same cup volume. Well said, Amber! Looking at the Simplicity 8229, the cup and frame patterns for the 34A and the 32B are not exactly the same. The 34 is slightly taller. I think that is okay because the difference is so small.

what is sister sizingwhat is sister sizingwhat is sister sizing

I also don’t suggest going up or down more than two sister sizes. As Amber and I show, the differences can get bigger the more sizes you jump.

Last, I HAVE read that sister sizing is a way for retailers to sell limited inventory instead of a bra that actually fits. Not sure if I believe that, but thought I’d mention.

Lace fabric in intro image is from Lacetime



  1. Reply


    Great post! As a bra technical designer, I frequently have to explain sister sizes to my merchant partners. We generally do not use sister sizes to sell limited SKUs/sizes, since the average consumer isn’t super savvy about sister sizing.
    Love your blog and your line- keep up the great work πŸ™‚

    • Reply


      Super interesting. Thanks for sharing! During my technical designer days, the designers didn’t have a clue what sister sizing was, so I understand when a RTW bra doesn’t offer it.

  2. Reply

    Tasha @ By Gum, By Golly

    As I’ve finally taken the plunge into bra-making, I’ve read a lot about sister sizing lately. In fact, a pattern I tried that was a bit small in the cup doesn’t come in a larger cup size for my band size, so I may try the sister size of the next size up and change the band accordingly!

    Now the thing that makes me kinda nuts about it in the *non* bra-making world is when pattern designers say things like “you may want to do an FBA if you’re larger than a C cup”. This has really hit home more while learning about bra making. Weeeell, a C on what band? If you *think* you wear a 36B because you haven’t realized yet that your band is too large and you should *actually* be wearing a 34C, you won’t think that applies to you, but maybe it does! I wish they’d approach it like “If there’s more than X inches different between your over bust and full bust, do an FBA”. But that’s another story… πŸ˜‰

    • Reply


      That would entail a shift in the way the sewing industry thinks. Now that bra making has become more popular, it could happen. Five years ago, however, I’d say forget about it!

      I totally agree that saying a “C” cup or larger requires an FBA is deceiving. We should be using the difference between over bust and full bust.

      If you need any help with your bra making adventures, don’t hesitate to holler!

    • Reply


      I have seen this in one of the Great British Sewing Bee books. I’m by no means an expert, but I tend to follow the advice in there for every pattern about the FBA. So far it seems to work, but I am still learning!

  3. Reply


    “Last, I HAVE read that sister sizing is a way for retailers to sell limited inventory instead of a bra that actually fits. Not sure if I believe that, but thought I’d mention.”

    I have experienced this myself. Clearly this varies from store to store and from fitter to fitter, but it definitely happens.

    • Reply


      I can’t say for sure that it happens, but I wouldn’t be surprised.

  4. Reply


    FBA??? Newbie here lol…. I’m so new, I’m just batter in the pan, but just wait till I get baked. And when I get my frosting, Oh My, I’ll be able to talk like all you pro’s…..

    • Reply


      Full Bust Adjustment… not to be confused with a Flat Bottom Adjustment! Also the SBA Small Bust Adjustment, and TNT (tried and true/tested) pattern and RTW (Ready to Wear, ie shop bought) clothing. I am also learning, and really struggled with the above!

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