Bra Making: What is Sister Sizing?

Sister sizes can help you use a bra pattern that may not have your exact size | Cloth Habit

Have you heard the term “sister sizing”? It’s a great term for explaining how bra sizes shift in band and cups. The basic idea is this: Sister sizing refers to the same cup but on a different size band. As a bra band size gets smaller, the same cup volume will have a bigger letter, and vice versa.

In other words, a B is not a B is not a B. A 36B is a full cup size larger than a 34B. So I’m not just “basically a C cup”. Nor is someone else “an F”.

The use of cup size in non-bra sewing patterns like blouses or dresses can make this a bit more confusing for sewists. Your cup size in dress patterns (like Simplicity’s Amazing Fit series, for instance) might be completely different from what you need in a bra. Dress patterns use the difference between the upper chest and full bust measurements to determine cup size, but bras are more dynamic in their sizing.

How to make sister sizing work for you

If you can’t find your size in a bra pattern, there’s a good chance the pattern has a “sister size”, and you can find the equivalent cup on another band, and simply alter the band to your preferred fit (a very easy adjustment!). For example a 28F on a 32 band would be a 32DD or 32E.

Choosing bra pattern sizes with the help of sister sizing | Cloth Habit

(Note: in the U.S. and UK “DD” is usually the cup size above “D”. There are other double letters, too, but most bra patterns use the normal alphabet. I personally find the normal alphabet less confusing!)

Obviously I didn’t fill out the chart with all the possible bra sizes and cup letters, but hopefully you get the idea!


There are some exceptions to the sister sizing “rules”, both in lingerie brands and in home sewing patterns. These exceptions do what I call “jump grading”, where the cups on the same band size will jump by two cup sizes after D cups. There are a few upscale brands that use this kind of size grading (Empreinte, Prima Donna) and Pin-up Girls sewing patterns use this method. I won’t go into details of how it works because it can get confusing unless you’re really into the nerdy ins and outs of bra grading.

Regardless of what size you start out with, there will always be a good chance that a bra pattern doesn’t fit the same way as your size in your favorite brand. That’s just the nature of the beast. There are many measurements that go into bra drafting, not just the full bust and ribcage measurement, and no measuring method is going to capture them perfectly.

Before you try online bra calculators remember that you are making a bra, which is a bit different than buying one. Find a starting size by using the pattern’s measuring suggestions, and then go from there to make a “you size”! Learning to fit is an essential part of bra making, period.

Some tips for fitting a bra pattern:

1. Fit a wire before you do anything else. A good-fitting wire solves other fitting problems further down the road.

2. Try to stick with the same band pattern (regardless of what size it is), tweaking it until it is firm enough for you, in the fabric you want to use.

3. You may end up needing to try different cup sizes. Sewing has a way of tuning our eye to fit issues we never noticed before and bras are no different. Nearly everyone I’ve fit has had to go up at least one cup sizes, usually two, from their “usual” size. And my top tip: it is better to start large in the cup and take in adjustments, than starting too small and trying to figure out where the room needs to be.

Have you had trouble fitting a bra pattern? I’ve got more pattern fitting posts in the pipeline so I’d love to hear what ails you.


  1. Mary Ellen says:

    Thank you, thank you, thank you for this article!! I know this to be true from my purchases but never have seen the concept addressed. I recently became interested in sewing my own lingerie as a have a larger band size and a smaller cup size. I can only have one or the other fit properly. So looking forward to creating undergarments that fit.

  2. Sister sizing is fine as far as it goes, but you run into issues when you try doing that too much–namely the straps and wires don’t sit where they should (IME). You can only take in a band so much, and bigger bands seem to assume wider shoulders, wider chests, and so on. Most of the bra bloggers actually say NOT do to do this. Obviously, some of this can be addressed (if you have the skill and patience) when sewing them for yourself, but if you aren’t…

    My biggest problem is the lack of options in larger cup sizes, and this is true whether I’m buying or sewing a bra. Pushups, plunges, strapless–all are hard to find, and a combination of those things is nearly impossible. *Especially* in bra patterns and affordable RTW options.

    • Amy says:

      I think that’s why I made a distinction between buying one and making one. My intended readers are sewists and not the bra blogging community, so doing a sister size in a bra pattern offers a bit more freedom. If having a really fine-tuned fit in an underwired bra is your goal, there are some adjustments that I think everyone should make from the outset, including strap placement and bridge shape/distance because no pattern is going to get that right for everyone and they are pattern aspects that can be easily customised.

  3. emily marie says:

    Great job illustrating how sister sizing works! You are so right, many many people believe that a B cup is a B cup. Once, my friend just did not believe me that my bra was a G cup. Yeah… 30G…
    I have had a bit of difficulty finding the size in RTW and always find something that I would tweak, maybe it’s time to start making my own bras. 🙂

  4. joelle says:

    thanks for writing that out in such a clear way! i was mystified for the longest time wether cup sizes in bras and in sewing pattern referred to the same thing… just like emily marie, i’ve shocked a few of my friends when i tell them what size bra i am wearing. i usually also tell them that they are probably wearing the wrong size…

  5. Roni says:

    Thank you for this post! A friend once told me something about it but I never fully understood the logic.
    My bra size is 75A (in the metric system) and I would really like to make myself a soft bra with no wire. This is something I can’t find in RTW in my size, all bras are too big for me and that’s getting a bit depressing. I would be happy if you could recommend patterns for small breasts that are still stylish and not juvenile.

    • scooter says:

      Roni, If you want a soft bra, the only modern patterns I know of are Oooh, Lulu! and Merckwaerdigh (BHS-10 I think). Merck has good, grown-up styling, but seems to run full in the center (which small busts usually aren’t) and rather projected (which small busts usually aren’t), but I’ve had good luck with their soft bras with some alterations for those things. Oooh, Lulu’s patterns are simple enough that I improvised something similar that works for me; their aesthetic may or may not appeal to you. For wired, you might try MakeBra, they have a “petite” bra pattern with smaller sizes, as does Pin-Up Girls, but both of them make the usual mistake of thinking small cup => small wire, whereas usually small cup sizes are shallow rather than projected and need wider wires than you’d expect based on cup volume. For a serious education on bust shapes and bra fitting, check out reddit/ABraThatFits!

    • Amy says:

      Hi Roni, nearly all the major bra patterns out there offer this size. It is usually comparable to a 34A in U.S./UK sizing–or in the ‘sister size’ 70B/32B. You can make an underwired pattern but not include the wire, which isn’t a weird thing to do. It’s a really common design in rtw bras. There are also quite a few soft exclusively cup bra patterns. (My upcoming pattern is going to be one!)

  6. LK says:

    That is absolutely fascinating! I want to tell someone but I’m surrounded by coworkers and that’s probably not a kosher chat subject.

  7. scooter says:

    Wait, did you just *ask* us to complain about our bra fitting woes? 🙂

    My personal recurring nightmare is a wire size and/or shape that doesn’t match what the bra pattern was drafted for. Do I use the appropriate band size for the wire and alter cup volume down, or use the right cup size and alter the band up? and How? What if the size is OK but the shape of the wire is not?

    On the subreddit ABraThatFits, conventional wisdom is that 80% of small busts are shallow, so why are all the underwired patterns for this size range so projected? Patterns that look great in larger sizes look strange or just plain don’t fit smaller sizes, over and over, and a big part of that is the shallow vs projected spectrum.

    • Amy says:

      Haha, I did! And you have some good questions, too.

      My suggestion is definitely to use the appropriate band/wire and alter the cup to fit. Projection is really a matter of how a certain cup volume fits a certain wire. The narrower the wire, the more the cup is naturally going to project. And the wider the wire, the more that same cup pattern will “flatten” for lack of a better word. It’s hard to explain this in words, without pictures, but the more projected/deep the cup, the more the cup pattern takes on a bit of an ovular shape. The more shallow, the more spherical. At some point I might show the difference in how a pattern could look from a patternmaking/fitting perspective. But also I’d love to show how to adjust a cup pattern to fit (your favorite) wire.

      That said, most patterns accommodate regular wires, because those are the most commonly available wires. Buying and manufacturing specific types of wires wholesale is extremely expensive, so home sewing retailers usually buy what they can get wholesale.

      Great ideas!

      • Scooter says:

        No, yeah, that makes total sense–spread the same volume over a wider base, and it gets flatter. I wasn’t clear in my original comment, but I was really thinking about a wire that is larger or smaller than expected for the breast volume, though shape variations could also wreak havoc, I’d imagine. I think you’ve said before that you use a wire that’s a size bigger than your bra size would indicate? Mine is like…four sizes bigger. I’m small enough I can wear wireless just fine, but darn it, I like to solve these problems, because the engineering is so interesting!

        Distribution of tissue on the chest seems to vary a lot too. Tall roots, short roots, narrow space between, wide space between, full on top, full on bottom…All more factors in the cup shape equation. Seems like maybe building a bridge would be easier. 🙂

      • Amy says:

        It’s really fun to play around with shapes and it sounds like you are fascinated by 3D thinking! Me too. Pattern fitting is such a great skill to pursue and stimulates a lot of that! You also might be interested in having a go at drafting your own cradle to mimic the wire that you want to use, if drafting peaks your curiosity. This article shows one way to do that (just the band drafting part). You can try different cups on that cradle and see how they turn out. I remember you asking me in email about how to adjust a cup to a cradle/wire of a different size and I think a tutorial on this is a great idea. I will plan one!

  8. Grace says:

    Thank you so much for this post! I know about sister sizing, and that I will have to avail myself of this knowledge to even start to fit a self made bra. This posts is just the encouragement and advise I need.

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