Letโs finally clear up the mystery of sister sizing. Iโve heard it all before: โItโs confusing,โ โIt doesnโt work for me,โ โI donโt get it.โ 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.

Letโs be realโbra shopping ranks right up there with going to the dental hygienist. You know itโs necessary and appreciate the benefits, but itโs not fun.ย
How often have you searched for your size in a big-box store only to find itโs 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 โ Iโve seen it all. We support women in need of all sizes, and finding a bra that fits is always easier said than done.
Iโve been through the wringer enough to pick up essential tips and tricks, including what I call the โtic-tac-toeโ method for decoding sister sizes.
But firstโฆ
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 itโs so uncomfortable or realize itโs simply not providing the support we need.
If youโre unsure whether itโs 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 youโre wearing the right size bra.
Decoding Sister Sizing: Elaineโs 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.ย
Weโve all been there: the bra doesnโt fit quite right, whether the cups are too big or too small, or the band feels too tight or loose.ย
A common mistake is assuming you can simply go up or down a band size and keep the same cup size, but thatโs not quite how it works.ย
Cup sizes are about proportion, as you can see from the chart belowโa C cup in one band size is not the same as a C cup in another band size.
Think of bra sizing like a game of tic-tac-toeโyou need to adjust both the band and cup size to get the perfect fit. Take a look at our chart below for help.
For example, if youโre wearing a 40D and know the D cup fits you, hereโs 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; youโre 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 bestโbecause 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 40Bโa tricky size because many stores donโt 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 38Cโone band size smaller and one cup size larger than her usual 40B.ย
The cup was a perfect fit, but the band felt a little too tight. Since a 42A wasnโt an option, we used bra extenders to make the band more comfortable, ensuring the cup size fit her just right.
My sisterโs experience with sister sizing was a huge relief after weโd struggled to find the right bra for so long.ย
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 youโre out bra shopping. As Iโve 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 youโre 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.ย
Here is my checklist for ensuring youโve got the right fit:
Adjust Straps & Bands:ย
- 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 doesnโt ride up and the cups still support.
- Bending: Make sure the bra doesnโt 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!
Iโm 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, Iโve introduced them to sizes they hadnโt considered before. I aim to create a โbra-voโ 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: Donโt throw them away!
The Bra Recyclers collects unwanted, gently used bras for recycling and donates them to women in need.ย
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!
Sister sizing can be so confusing, but this breakdown makes it a lot easier to understand! Itโs great to have options when the perfect band or cup size isnโt available.