Bra measurements are just a starting point, there are no one size fits perfectly for everyone as all bodies come in different shapes and sizes which is also true when it comes to breast shape and size. 

Sister sizes are a great tool in helping you find a fit that you find works best for you. Now that you have the basics down for how to measure yourself, let’s talk about sister sizes. 

Sister Sizes 101

Sister sizes are bra sizes that have the same cup volume but different band sizes. In other words, sister sizes are bras that have different numbers and letters but can provide a similar fit.

Sister sizes revolve around cup sizes so it’s a good guideline for where to start. Since cup size is measured by volume and band size is measured by length, the general guide is that if you go up a band size, you go down a cup size and vice versa, if you go down a band size, you will go up a cup size.

Cozy Plunge in Classic Blue

How to find your sister size?

  1. To go up in band size and maintain the same cup volume, go down in cup size. For example, if you are a 34B and want to try a looser band, you can try a 36A.

  2. To go down in band size and maintain the same cup volume, go up in cup size. For example, if you are a 36D and want a snugger band, you can try a 34DD.

You may find that your band is a little snug but your cup fits well, you would then size up a band size and down a cup size, for example, if you are a 32C is a sister size of 34B. Same is true if your band size is too big and your cup size is correct, you should size down in the band and up in a cup, for example, if you are a 36C your sister size would be a 34D.

How to Measure Your Bra Size - Floatley

How to use sister sizes to fine-tune your bra fit and improve overall comfort

It's important to note that while sister sizes can provide a similar fit, the overall comfort and support may still vary due to differences in band tension and shape. It's always recommended to try on different sizes and styles to find the best fit for your unique body shape and preferences.

In addition to finding your Sister Sizes, learn new expert tips on when to replace your bras and how to care for your bras! 


Using sister sizes can help you fine-tune your bra fit and improve overall comfort.

  1. Adjust the band size: If you find that your band feels too tight or too loose, but the cup size seems to be the right fit, you can try sister sizes with different band sizes. Remember, as you go up in band size, you should go down in cup size, and vice versa. For example, if you currently wear a 34C and the band feels too tight, you can try a sister size of 36B to get a more comfortable band fit while maintaining the same cup volume.

  2. Fine-tune the cup size: If you feel like the cups are slightly too small or too large, but the band fits well, you can experiment with sister sizes to adjust the cup size. Going up or down a cup size while keeping the same band size can help achieve a better cup fit. For instance, if you wear a 36D and the cups feel slightly too big, you can try a sister size of 36C for a slightly smaller cup volume.

  3. Explore sister sizes in different brands or styles: Sister sizes can be particularly useful when trying bras from different brands or styles. Not all bras are created equal, and sizes may vary. If you find a bra you love but your usual size isn't available, try exploring sister sizes within that brand or style. You might discover that a sister size provides a better fit and improved comfort.

  4. Consider the purpose and style of the bra: Different bra styles and purposes may require slight adjustments in sizing. For example, if you're looking for a sports bra with a snug and supportive band, you might consider going down a band size while maintaining the same cup size. On the other hand, if you prefer a more relaxed and comfortable fit for a bralette, you could try going up a band size while adjusting the cup size accordingly.

Remember, while sister sizes can be helpful, they are not an exact science. The fit can still vary depending on your individual body shape, bra brand, and style. It's essential to try on different sizes and styles to find what feels the most comfortable and supportive for you.

Floatley Team