Sister Sizing Explained: When and How to Use It

A sister size is a bra size with the same cup volume as your true size but a different band and cup letter combination — for example, 32D and 34C hold the same amount of breast tissue. Sister sizing is useful when your exact size is out of stock or a specific style runs differently, but it is a substitute, not a replacement for knowing your true measured size.

How Sister Sizing Works

Moving up one band size and down one cup letter, or down one band size and up one cup letter, keeps the cup volume constant. So if your true size is 32D:

• Sister size up: 34C (larger band, one cup letter down)
• Sister size down: 30DD/E (smaller band, one cup letter up)

Both hold roughly the same cup volume as a 32D, but the band fit will feel different — looser in the 34C, tighter in the 30DD/E.

When Sister Sizing Is Useful

• Your exact size is sold out and you need a close alternative.
• A specific style or brand runs small or large, and a sister size compensates.
• You are between two band sizes and want to compare the fit of both directions.

When Sister Sizing Causes Problems

Using a sister size as a permanent substitute for your true size, rather than an occasional workaround, often produces a band that is too loose paired with a cup that is too small — the same failure pattern caused by the plus-4 measuring method. This leads to straps digging in, bands riding up, and support coming from the wrong place. Sister sizing should be a temporary fix, not a standing size decision.

Sister Sizing Is Not the Same as Guessing

Sister sizing only works reliably once you already know your true measured size. Start with the rib cage method to establish your actual band and cup size, and use sister sizing only as a secondary tool from there.

Frequently Asked Questions

What is a sister size in bra sizing?

A sister size is a different band and cup combination that holds the same cup volume as your true size, for example 32D and 34C are sister sizes.

Should I just buy my sister size instead of my true size?

No, not as a regular practice. Sister sizing is useful when your true size is unavailable or a specific style fits unusually, but using it permanently tends to produce a looser band than you need.

How do I find my sister size?

Move one band size up and one cup letter down, or one band size down and one cup letter up, from your true measured size.

Why would I need a sister size at all?

Brands and styles vary slightly in how they are cut and constructed, so a sister size can help you find a comparable fit when your exact labeled size is out of stock or runs differently than expected.

Find your true size first with BRABAR's Fit Guide and Bra Size Calculator, then explore the full range of band and cup sizes.