Bras: What is Sister Sizing?

Feb 20, 2017 | Hurray for Lingerie, Underneath | 23 comments

I have a lot of siblings. Six brothers and one sister, actually. Having a sister is great sometimes. Sisters know you to the core, support you, share their clothes, and tag you in “only we will get this” memes on Instagram. And while sisters can be very similar, they are also very different.

My sister and I are actually stepsisters. And while we call each other “sister” (no, really. When we address each other on cards and on the phone, it’s just as “sister”), we really have a lot in common and a lot of differences. The same goes for “sister sizing” when it comes to bras.

I’ve been teaching women about bras and bra fitting for years – since 2005! And one of the things that still surprises me is that many women are still surprised about how bra sizing works. The concept of “sister sizing” seems simple, and is really important to understand. It’s a bra sizing concept that you’ll use to adjust your size, try different bras, and find a well fitting, comfortable bra.

What is Sister Sizing? 

“Sister Sizing” is the name that the lingerie and bra fitting industry use to discuss similar bra sizes with different bra band sizes. They are “related”, if you will. Usually, calculating your “sister size” is the result of changing the band size, which then affects the cup size. Knowing how “sister sizing” works is helpful when you are trying on bras. This basic knowledge will help you change bra band sizes and know how to adjust the cup size in order to get the fit you’re looking for.

“Sister Sizing” is the reason that I suggest two different bra size starting points when you calculate your bra size. (More resources for this coming soon on Hurray Kimmay!)

Here’s an example: Let’s say you are trying on a bra in size 32C and wanted to adjust the band for a better fit. If the 32 is too tight, you’d adjust the band up to a 34. If the band is too loose, you’d adjust the band down to a 30. Seems simple, right? But what would you do with that “C” cup? Keep reading!

How does Sister Sizing work? 

First, it’s helpful to know how bra sizing works in general. Cup sizes usually start at an A and use the English alphabet to go up to about an N cup. American, European, and UK sizing differ on the size they use to label their bras. {For more on the variations across countries, you can read my article on bra sizing on About and stay tuned for more information here.} For our purposes here, I’m keeping it simple and using the US band and cup sizing method (which in the cup sizes noted here, are consistent across the UK, too. Hurray!)

Okay so let’s dive in. Did you know that a cup size is relative to the band size? That means that a C cup, for example, is not the same volume, or “size” across different bra bands, like a 30, 32, and 34 band. If you’ve ever met someone who wears a 40C and someone who wears a 32C, for example, you may have noticed that the woman wearing a 40C has a noticeably fuller bust than that of a woman who wears a 32C. That’s because the cup volume for each cup size adjusts with the band size. The cup size volume is one cup size fuller for each bigger band size. So not only is a woman who wears a 40C wearing a band size that is larger, that C cup that she is wearing is bigger, too. It’s all relative to the band size.

C cup examples bra sizing Hurray Kimmay blog

Here’s that simple example from above. If we are adjusting the size of the same bra, here’s how the cup size would differ across band sizes:

  • A 32C is one cup size bigger than a 30C.
  • A 32C is one cup size smaller than a 34C.
  • That also means a 34 is two cup sizes bigger than a 30C, and so on.

Sister Sizing examples Hurray Kimmay blog

So what if you just want to adjust your band size and keep the cup size the same? To get the same cup size volume across band sizes, you would adjust the cup size up or down one for each band size change:

  • A 30D is the same cup size as a 32C, or a 34B… and so on.
  • The difference here is the band size, or length of the band. (And possibly how the cups are placed on the band. They tend to be spread further apart as the band gets bigger, typically). The cup volume remains the same.

The basic rule to remember is “up in the band, down in the cup” or “down in the band, up in the cup” to keep the same cup size volume.

  • That means if you adjust your band size up one (from 32 to 34, for example), you’ll need to adjust your cup down one size (from C to B, for example) to get the same cup size volume.
  • Or, if you’re adjusting your band size down one (from a 32 to 30, for example) then you’ll adjust your cup size up (from C to D, for example) to get the same cup size volume.
  • If you want to adjust your band size AND cup size up one each, you’d go up one in the band (32 to 34, for example) and keep the cup size the same (C to C, for example).
  • If you want to adjust your band size AND cup size down one each, you’d go down one in the band (32 to 30, for example) and keep the cup size the same (C to C, for example).

Tricky Sister Sizing rules

The purpose of this post is to introduce the concept of “sister sizing” and help you navigate your next bra shopping experience with greater ease. Hurray! So I want to touch lightly on how the simple concept of “sister sizing” can get a bit complicated.

As I mentioned in a previous blog post on Why 9 Different Bra Sizes All Fit, the world of bra fitting is not standardized or an exact science. The manner in which bras are sized changes across brands, styles, and countries. The good news is that due to the way bra patterns are made, the concept of “sister sizing” is safely applicable to most bra sizing methods in the world. In other words, whether you are shopping a brand from the UK, France, or the USA, if you’re adjusting your bra size in the same exact bra style, you can safely adjust your band and cup using the method above with a great deal of accuracy. Hurray!

However, once you start to adjust your cup across brands and styles it can get tricky. That’s why it’s good to use this method as a rule of thumb, but keep in mind that assessing the fit of your bra is the most important step toward finding a fit that fits YOU best.

Kimmay with tape measure hurray kimmay blog

Avoid the Sister Size trap

Once I explain how “sister sizing” works, my bra fitting clients often say “Oh, great! Since I was measured for a 30D, that means I can buy a 32C or a 34B and wear that, too!” And while, technically that cup size will fit, that doesn’t mean a bra in one of her “sister sizes” is going to be the best fit. In my Top Five Bra Fit Tips, I explain how to tell if your bra fits, and that having a snug bra band is #1 on the list. Never sacrifice a good fit!  A bra that fits well, feels comfortable, and looks great can give you the confidence to take on the world. A bra that fits well is a great way to say hurray inside, outside, and underneath™.

If you find a bra in a “sister size”, and it isn’t the best fit for you, skip it. There is no need to settle. And if a bra fitter or salesperson suggests that the “sister size” is “the same thing”, keep in mind how actual sisters are. They may be similar, but they are not “the same”. And while one is not better than the other, it is very possible that – when it comes to bras – one is a better fit for you than another.


“Sister sizing” is a helpful concept to know when it comes to finding a bra that fits well and feels great. And, it’s an important reminder that numbers are information, not a definition. That’s why I created the #MoreThanMyNumbers project (coming soon!) Because knowing your bra size starting point and your “sister sizes” is helpful information, but you are so much more than the size on a tag or any other number.

Your Turn: What are your bra size starting points? Have you ever heard of the “sister size” concept? How can you apply this knowledge to your next bra shopping trip? What questions do you have for me? Leave a comment below and share your findings with us! And if you want to dive deeper and finally understand how to find a comfortable bra without wasting money on those you dread wearing, check out my Bra Confidence & Comfort Course! You’ll get all the information you need (in an easy, fun way) to say hurray.

xoxo kimmay

Notes: As always, I never photoshop/airbrush/edit my body. Because I believe every body is beautiful and deserves to be loved. Photos by Laura Boyd of Own Your Sexy. I’m wearing a Natori Feathers bra in 32DD, which I purchased with my own funds. This is not a sponsored post 🙂