Bra Math: Cracking the Code on Sister Sizes for the Right Fit

Let’s finally clear up the mystery of sister sizing. I’ve heard it all before: “It’s confusing,” “It doesn’t work for me,” “I don’t get it.” And honestly, I used to feel the same way.

My first introduction to sister sizing was 15 years ago, thanks to a blog by Tomima at HerRoom. At first, I found it confusing and hard to wrap my head around. But as I gained more experience with bra fitting, sister sizing became my go-to tool for helping women find the perfect fit.

Let’s be real—bra shopping ranks right up there with going to the dental hygienist. You know it’s necessary and appreciate the benefits, but it’s not fun. 

How often have you searched for your size in a big-box store only to find it’s unavailable or on special order? Especially if your cup size is G and up or on the other end, an A or B.

As the CEO of The Bra Recyclers for over 15 years – I’ve seen it all. We support women in need of all sizes, and finding a bra that fits is always easier said than done.

I’ve been through the wringer enough to pick up essential tips and tricks, including what I call the “tic-tac-toe” method for decoding sister sizes.

But first…

When is it Time to Change Your Bra Size?

Sooner or later, we all reach a point where we want to get rid of that annoying bra because it’s so uncomfortable or realize it’s simply not providing the support we need.

If you’re unsure whether it’s time to change your bra size, here are some signs:

• The straps are digging into the shoulders or falling off.
• The band is riding up or feeling too tight.
• The band is too loose, even on the tightest setting.
• Gaps or gaping occur, or cups overflow.

Pregnancy, breastfeeding, hormonal changes, and fluctuations in weight are all important factors to consider when determining if you’re wearing the right size bra.

Decoding Sister Sizing: Elaine’s Practical Guide

When considering sister sizes, think of it as a way to expand your options when shopping for bras. Sister sizing has been beneficial when looking for plus size bras, allowing for a wider range of choices for myself and the women I help fit.

In simple terms, sister sizing is about matching the correct band size with your cup volume/size. 

We’ve all been there: the bra doesn’t fit quite right, whether the cups are too big or too small, or the band feels too tight or loose. 

A common mistake is assuming you can simply go up or down a band size and keep the same cup size, but that’s not quite how it works. 

Cup sizes are about proportion, as you can see from the chart below—a C cup in one band size is not the same as a C cup in another band size.

© The Bra Recyclers

Think of bra sizing like a game of tic-tac-toe—you need to adjust both the band and cup size to get the perfect fit. Take a look at our chart below for help.

© The Bra Recyclers

For example, if you’re wearing a 40D and know the D cup fits you, here’s how to find your sister size:

• If the band feels loose, go down a size. For a 40D, your sister size would be 38DD/E. The rule is that when you go down a band size, you go up a cup size. The cup volume stays the same; you’re adjusting the band supporting it.
  
• If the band feels tight, go up a size. In sister sizing, a 40D would become 42C.

It might still seem a bit confusing, but sister sizing can really help when shopping for bras. Since different styles and brands fit differently, I always recommend trying on a few sizes to find what works best—because not all bras are equal in size.

A Tale of Two Sisters (and One Perfect Fit!)

Let me share a story about how sister sizing came to the rescue for my real sister. I went bra shopping with my sister, who wears a 40B—a tricky size because many stores don’t carry B cups in plus sizes.

After several attempts, we finally visited a local lingerie boutique, Belle Lacet, where the fitter was well-versed in sister sizing.

Thanks to her expertise, we found a bra my sister loved in a size 38C—one band size smaller and one cup size larger than her usual 40B. 

The cup was a perfect fit, but the band felt a little too tight. Since a 42A wasn’t an option, we used bra extenders to make the band more comfortable, ensuring the cup size fit her just right.

My sister’s experience with sister sizing was a huge relief after we’d struggled to find the right bra for so long. 

Tips and Tricks for the Right Fit

Example: this lady might want to consider a wider band, as the current fit around her back creates an uncomfortable bulge.

I get that some folks are still skeptical about sister sizing, but it can be a handy guideline when you’re out bra shopping. As I’ve mentioned, knowing your bra size is the most important thing.

You can do a DIY bra fitting at home or visit a local lingerie boutique for a professional fitting. (check out my guide here!)

If you’re looking for more personalized help, we have a great community of Recycling Ambassadors on The Bra Recyclers. They can assist you in finding the perfect bra for your body type and style preferences. 

Here is my checklist for ensuring you’ve got the right fit:

Adjust Straps & Bands: 

• Make sure the straps are snug but not digging in.
• The band should sit comfortably, parallel to the ground.

Check the Fit:

• Standing: Ensure the bra stays in place and the cups fit well.
• Sitting: Check that the band doesn’t ride up and the cups still support.
• Bending: Make sure the bra doesn’t shift or cause spillage.

Feel for Comfort:

• Move around and see if the bra remains comfortable and supportive.

Example: For a more comfortable fit, consider a broader band that avoids pinching or bulging, as demonstrated here.

Sis, You Got This!

I’m not here to pretend that sister sizing is a walk in the park, but following this guide can be as easy as tic-tac-toe.

As a bra fitter, my mission is to ensure that I can offer you the most comfortable options tailored to your body type and desired style. Sister sizing allows me to recommend alternatives and expand your choices.

For plus size or petite women and girls struggling to find suitable styles, I’ve introduced them to sizes they hadn’t considered before. I aim to create a “bra-vo” moment for everyone I fit.

What to Do with Your Ill-Fitting Bras

I hope this guide has helped you find your sister size! If you upgrade your bras, you might wonder what to do with those that no longer fit.

Hint: Don’t throw them away!

The Bra Recyclers collects unwanted, gently used bras for recycling and donates them to women in need. 

For many women in underserved communities, a bra is a luxury they cannot afford, and particularly for plus size women, bras can offer modesty and comfort while facing a vulnerable situation.

You will not only support women in need but also help reduce textile waste by supporting global sustainability initiatives!