Getting To Know Your Sister Bra Sizes

Sister Bra Sizes
Wouldn’t it be great if all bras of the same size always fit the same? And jeans, too! Fortunately in the case of bras, “sister” sizes exist. I’ve always heard this mentioned, but have never fully understood how sister bra sizes work or what mine are. HerRoom is full of great bra fitting advice, including Your ‘Sister’ Bra Sizes & Your Bra Size Neighborhood.

This seemingly simple article of clothing can be very complicated; it is actually pretty complicated and complex. The anatomy of bra consists of many parts and pieces that impact a bra’s effect on the breast—supportive qualities, comfort and longevity of the bra for example. We wear bras every single day—well, most days—and barely have an understanding of how they’re made or work. In unison, all these parts provide the lift and coverage we need.

When it comes to selecting a bra size, it’s important to consider band size in relation to cup size. The two factors that impact your true size and sister sizes are: cup size and band measurement. Going up a band size means you need to go down a cup size and cup sizes are not equal across all band sizes. It’s no wonder it’s difficult for us ladies to easily find a bra that fits—it’s actually very confusing.

Here are a few insights to bra sizing

Breasts have two measurements: diameter and projection. The distance the breast tissue projects from your chest wall is the projection and overall volume is measured by diameter.

Cup size selection—choosing A, B, or G, for example—reflects the projection measurement. Band size selection—34, 36, 38 and so on—denotes the diameter measurement, which is predetermined by the bra manufacturer and can vary across bra brands. So, changing your band size also changes your cup size. See the chart below for a depiction of how the sister size change. Find your size and follow the diagonals.

Still puzzled by the bra size pieces? You’re not the only one. Visit our bra fitting resources like the Know Your Breast Bra Finder™. This will take you step-by-step in finding the best bras for your unique breast shapes. Fitting comments and Universal Size information can be found in all descriptions of bras offered on HerRoom.

Have you had success with sister bra sizes? Tell us your experience in the comments.

Meghan Barich is a writer, the storyteller at and a proud Hoosier. When she’s not crafting the next story, you can find her nose-in-a-book or walking her dog, Moose.

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