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Sister Sizes: Why Every Woman Should Know for Bra Fitting

Have you ever encountered the frustration of finding that the bra you desire is unavailable in your size? Do you often feel that your current bra doesn't fit properly, whether it's the band or cups, and you're in need of a better fit? If you can relate to these situations, it's important for you to familiarize yourself with bra sister sizes, as they can greatly assist you in finding the right fit for your bra.

The Bra Band: Bust Ratio Explained

To understand the sister sizes, it is important to know the bra band and cup correlation that is the foundation for bra engineer and design. This has been a huge myth for most women. To resolve this myth, you first need to know the following facts:

  • Bra sizes are in essence ratios - band: bust ratios.
  • For the same cup size, cup volume (how much breast tissue the bra can hold) increases while band size increases. This is the most common misconception most women have, as they believe the the same cup size holds the same breasts volume among all sizes of women.
  • For the same band size, cup volume (how much breast tissue the bra can hold) increases while cup size increases.

What Are Bra Sister Sizes

Bra sister sizes are a group of sizes that provide the same cup volume.  With sister sizing, you need not compromise the fit of the cup (because the cup volumes are the same) by just go UP in the band size and DOWN in the cup size.  It can also be the other way around – DOWN in the band size and UP in the cup size. For example, if you are a size 32C, you might find sizes 30D, 34B and 36A to be fitting.  In simple terms, most likely sister sizes can be substitutes to your true bra measurement.

For a better understand about bra band and bust ratio and sister sizes, please refer the following chart for details.

Bra Sister Sizes | FinallyBra

Bra Sister Size Chart

Below is a sister size chart from which you can find bra sizes that is related to your actual size. You should start with the size for which you have been measured, and the size that you are looking for in the lingerie shop. Moving along the same row, you will find the sister sizes that have the same cup volume.

Bra Sister Size Chart | FinallyBra
Each Row of Sizes Share the Same Cup Volume


Bra Sister Sizes and Bra Fitting

Now let us explain why sister sizes are a key for bra fitting by solving the issues most women have.

Your Size of Bra Is Unavailable

If you can’t find your size, use the next closest sister sizes. As you move along the sister sizes, the band will become looser or tighter on your body, depending on whether you are going up or down band sizes. Too loose of a band fit will result in a bra ride up and less support for the breasts. Too tight of a band fit will cause body fat bulges and extreme discomfort. As such, we recommend the next closest sister sizes.

Your Current Bra Does Not Fit

Here are three scenarios for your current size:

  • Your cup fits, but band doesn’t: both the band and cup sizes need to be adjusted. Say if your current size is 36C, instead of adjusting to a size 38C (if 36 is too tight) or 34C (if 36 is too lose), your correct size should be a 38B or 34D in order to keep the same cup volume.
  • Your band fits, but cups don’t: keep the same band size but go up (if cups are too small) or down (if cups are too big) the cup sizes. Say if your current size is 36C, and C cups don’t fit, your correct size should be a 36D (if C is too small) or 36B (if C is too big).
  • Neither your band nor cups fit: adjust the band size basing on your need (up or down band size) first then adjust the cup size, most likely the cup size stays the same. Say if your current size is 36C, and you need a band size 38. Then most likely 38C is the correct size as it also has a bigger cup volume than 36C.

Now you know the sister sizes and hopefully this information helps alleviate some of the frustration for your bra shopping. Although the sister sizing is claimed to be accurate, it is possible to find exceptions, and for good reason. Women’s bodies are not the same, so sister sizing may not always work.  The best option to get the best fitting bra is to get fitted and try on the various types of bras that catch your fancy.