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👯‍♀️ Sister Sizes Explained: Same Volume, Different Band

Sister sizes share the same cup volume but use a different band. They are the most useful trick in bra fitting — and almost no one knows about them.

What is a sister size?

When you go DOWN one band, you go UP one cup to keep the same volume. Going UP one band means going DOWN one cup. So 32D, 34C, and 30DD all have the same cup volume — just different rib-cage dimensions.

Why they matter

If your bra cup fits but the band feels tight, sister-size up: same cup volume, looser band. If the band fits but cups overflow, sister-size down: tighter band, bigger cup.

Sister-size example

Starting at 34C: sister sizes are 32D (down a band, up a cup) and 36B (up a band, down a cup). All three have the same cup volume.

Frequently Asked Questions

Are sister sizes interchangeable?
Cup volume yes. Band fit no. Sister sizes are useful when you cannot find your true size or when a brand runs slightly differently.
How many sister sizes are there per size?
Two — one up, one down. You can chain further but the volume drifts.

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