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What Does Sister Size Mean in Bras?

Published in Bra Sizing 3 mins read

Sister sizes in bras refer to different bra size combinations that share the same cup volume, even though their band and cup letters are different. This concept provides flexibility when your usual size isn't available or when you're looking for a slightly different fit.

Understanding the Concept of Sister Sizes

At its core, sister sizing is based on the principle that cup size is relative to band size. A "C" cup on a 32 band does not have the same volume as a "C" cup on a 38 band. The cup volume increases as the band size increases, even if the cup letter remains the same. Sister sizing works by adjusting both the band and cup letter simultaneously to maintain a consistent cup volume.

  • Decreasing Band Size: If you go down one band size, you should go up one cup letter to maintain the same cup volume. For example, if you typically wear a 34C, decreasing the band to 32 would mean increasing the cup to D, making 32D a sister size to 34C.
  • Increasing Band Size: Conversely, if you go up one band size, you should go down one cup letter to keep the cup volume consistent. So, for a 34C, increasing the band to 36 would mean decreasing the cup to B, making 36B a sister size.

This adjustment ensures the amount of breast tissue the cup can hold remains the same, even though the circumference of the band changes.

How Sister Sizing Works in Practice

Let's use a common example to illustrate sister sizing, such as a 32C bra size.

Original Size Sister Size (Down Band, Up Cup) Sister Size (Up Band, Down Cup) Cup Volume Equivalence
32C 30D 34B All have similar cup volume

In this scenario:

  • A 30D bra offers a tighter band fit than a 32C, but with a cup designed to hold the same volume of breast tissue.
  • A 34B bra offers a looser band fit than a 32C, while still accommodating the same cup volume.

When to Consider Sister Sizes

Sister sizing can be a helpful tool in several situations:

  • Between Sizes: If you feel like you're "between" two sizes and one feels too tight and the other too loose, a sister size might offer the perfect compromise.
  • Preferred Band Fit: Some individuals prefer a snugger or looser band fit. Sister sizing allows you to adjust the band without compromising the cup fit.
  • Availability Issues: If your exact size isn't available in a particular style or brand, a sister size might be a viable alternative.
  • Brand Variation: Bra sizing can vary slightly between brands. If one brand's 34C feels off, its sister size (e.g., 32D or 36B) might offer a better fit in that specific brand.

Important Tip for Finding Your Best Fit

While sister sizing offers flexibility, it's crucial to first determine your true bra size through a proper fitting. Many lingerie stores and specialty shops offer professional fitting services, which can help you establish your baseline size. Once you know your accurate starting size, exploring sister sizes becomes a much more effective strategy for finding bras that offer maximum comfort and support.

For more information on finding your perfect bra fit, consider consulting resources from reputable lingerie experts or visiting a local bra fitting specialist.