Sister Sizes: What Are They & Why You Should Know About Them

Renata Black, EBY Co-Founder & CEO

Finding a bra that you love only to discover that it’s no longer available in your size can be frustrating. 

Bras come in a wide range of designs and fits, with each one boasting a different function, from push-ups to bralettes. Therefore, once you’ve found the perfect combination of design and function, you shouldn’t let the wrong size hold you back!

Unlike other garments, bra sizes don’t become larger as the size increases or smaller as the size decreases. Bras are unique in measurements, as a range of sizes similar to your exact bra size can be altered to fit your body’s unique proportions. These are what are known as sister sizes.

What are Sister Sizes? 

Sister sizes are equivalent bra sizes that exist in groups according to the cup volume. You are likely to already be familiar with sister sizing if you have ever tried a new brand of bra, as most bras fit slightly differently depending on fabric, cup shapes, underwires, and manufacturing procedures. 

As a result, your exact bra sizing may be different in other stores, which is due to the adaptability of bra sister sizes.

Sister sizes aid the process of finding the perfect bra size that suits both the unique style of the bra, as well as your particular breast shape. While the actual cup volume remains the same in bra sister sizes, the band size and cup letter change to achieve maximum comfort.

Why Should You Know About Sister Sizes? 

Finding the right bra size that fits the shape, size, and curvature of your breasts, all while having comfortably fitting bands, can be a real struggle. 

One of the biggest benefits of bra sizes is that they can be adjusted to suit your body and comfort. Therefore, understanding your sister size is essential when you can’t seem to find a bra size that has perfectly fitting cups and bands.

Moreover, sister sizes allow you to shop from brands that don’t cater to your exact size. They also come in handy when a store has run out of your usual size.

Additionally, physical changes in the body, such as pregnancy and hormonal changes, can cause many women to increase their bra size. In such cases, sister sizes are the best options to try.

How to Calculate Sister Sizes? 

To calculate sister sizes, you will first need to know your cup size. Essentially, you will want to try on as many different size combinations as you can to find the perfect fit.

Below, we have explained the three scenarios in which you can work out your sister sizes most easily.

Bra band fits, but cups don’t:

If cups feel small, select the next larger cup size. If cups feel big, simply select a smaller cup size. 

Cups fit, but the band doesn’t:

This one may be a bit tricky to remember at first, but when going up a band size, you will need to go down in cup size. When going down a band size, you need to go up in cup size. 

Neither cups nor bands fit: 

If both are too small, go up in cup size and band size. If both are too large, go down in both cup size and band size.

Sister Sizes Chart 

The following chart demonstrates how sister sizes work. The sizes on the left-hand side represent a larger cup but a smaller band, whereas the right side shows smaller cups with bigger bands.

Sister Sizes Chart - EBY

The average American bra size is 34DD, so we will use this size to help explain the chart. After locating 34DD, the sister sizes can be found along the same row. For example, 30F and 38C are sister sizes!

When looking for bras and bralettes, remember to check this chart for a better and more thorough shopping experience.

How To Check If Your Bra Fits Perfectly?

To check if your bra fits correctly, make sure you have:

  • No gaps or spillage in the cup area.
  • Straps that don’t fall or hurt your shoulders.
  • An underwire that doesn’t dig into your chest.


Eby prides itself on supporting women through great-fitting, ethical lingerie! With our inclusive bralettes line, we endeavor to provide women with beautiful, functional underwear for optimal comfort.

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