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Bras Sister Sizes: What Are They?

Ever heard the term' sister size' related to bras and hadn't a clue what was meant by that phrase? Or perhaps you know what it means, but you've wondered how two bras with entirely different sizes can still fit so similarly?

Our team is here to show you how sister sizes work and what the term means. So, buckle up, grab your favorite cup of tea, and let's explore the fascinating world of bra sister sizes, including the ever-important true cup size. Here we go! ☕️

What is sister-sizing? Sister sizing is the concept that bras with different band and cup sizes can share the same overall cup volume.

This is because cup size is relational to band size. So while 34D and 34C are two different cups with two different volumes, a 34D and a 32DD, and a 36C all share the same cup volume, despite having different band sizes because of the way sister sizing works.

When you go up a cup size while going down a band size or the inverse, going up a band size and going down a cup size, you are playing around with sister sizes.

Another way to put it is that a sister size is another size bra option whose cup volume + band measurement balance out to share roughly the same volume as its mate with a slightly bigger or slightly smaller band size.

Confused? Don't worry; let's break it down a bit more.

A visual showing how different band and cups sizes can have the same cup volume.

How does sister sizing work? Generally speaking, we have something called a 'true' cup in bra-making. To be considered a true cup, it must be a cup that is designed to sit on a 34” band. True cups help create a size baseline. Theoretically, if you started your size range by making a 34AA cup to a 34K, your size range would cross the span of almost every size imaginable. The true cups mostly make up the full-size range of most bras we see today, as these are the cups that grade across most bra sizes.

Why do the true cups all sit on a 34" band? Part of the reason is that 34B tends to be the base size for most bra ranges, while 34DD is usually a grade check size. The true cup is meant to sit in the middle of the size range so that we can easily size up or down to make the benefit of sister sizing work.

When you go up in the band size but down in the cup size (or vice versa), the actual cup volume remains the same. This means that if a 34C bra feels too tight around the band, you might find comfort in a 36B without sacrificing the fit of the cup. Keep in mind, though, sister sizing isn't a one-size-fits-all solution. It's merely a starting point to help you find your own ideal fit.

Take the below Bra Sizing Matrix to better understand. The ‘True Cup’ sizes are all outlined in bold black lines running down the 34 band. All bra sister sizes follow the same-colored diagonal blocks.

A size chart explaining bra sister sizes


Size 34DDD can convert to be used in any of the below sizes:

46AA = 44A = 42B = 40C = 38D = 36DD = 34DDD = 32G = 30H = 28I = 26J

Part of why this true cup talk is so important is because of molded cups. In the bra world, we're split a little more than half and half, with 60% of bras being molded cups with the other 40% being softer, non-foam cup options. In bras, we tend to use a lot of molded cups for that smooth t-shirt bra effect which has become oh-so-popular over the past two decades. I’m still researching this subject, but I believe the molded cup is why we’ve started relying much more on sister sizing in recent years.

Mold used to make molded bra cups

When you're using molded cups, it's incredibly costly to develop each mold, so as a result, brands try to use sister sizing whenever possible to avoid making a different mold for every single size in the range. If we made a mold for every size, we’d have 100+ molds for a complete size range instead of only a small handful. Sister-sizing the molds means we can use far fewer, more like 8 to 12, while accomplishing the same goal of having many sizing options to select from.

For clarity, soft cups and unlined styles can also be sister-sized. Still, because these styles aren’t forced to work within a commercialized molded pad, all the details are flexible within the pattern itself, so the fit of these garments can be a little more perfected usually.

To determine your true cup size, you'll need to know both your band size and your bust measurement. Find our earlier blog post on bra sizing here.

Remember, this is just a starting point, and every brand might have slight variations in sizing. Therefore, it's always a good idea to try different brands and styles to find the perfect fit for your unique shape.

Whether you're looking for a more comfortable fit or exploring different styles, understanding sister sizing will make the journey much smoother.

As you try on different bras, remember that sister sizing is a guide, not a rule. Always prioritize comfort and support, and don't be afraid to ask for help from a professional bra fitter if you're unsure. Nordstrom is a good department store option, but local bra boutiques might be best for a personalized experience.

And there you have it, now you know the secret behind sister sizing and how to use it. So get ready to try some new bra sizes and remember: the perfect fit is out there waiting for you!



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