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Bra Sister Size Calculator

Find bra sister sizes with the same cup volume across band sizes, so you can shop fit instead of label.

Sister sizes

Enter your current size to see equivalents.

Your sister sizes
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Pick your size to see equivalents.

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What sister sizes are

Cup volume depends on band and cup letter together, so a 34D holds the same volume as a 32DD or a 36C. Going down a band means going up a cup letter to keep the volume, and going up a band means going down a cup letter.

When to sister size

Sister sizes help when your usual band is out of stock or when the band fits slightly off. A tighter band gives firmer support while a looser band trades support for comfort, with the same cup volume either way.

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Bra Sister Sizes FAQs

What is my sister size for 34D?
32DD and 36C, plus 30E and 38B at the extremes. Enter 34D above to see the full equivalent list.
Do sister sizes fit exactly the same?
The cup volume matches but the fit differs: smaller bands feel firmer and the wire width changes slightly. Always try before committing.
Why does the cup letter change with the band?
Cup letters measure the difference between bust and band, so the same letter on a bigger band is a bigger cup. Sister sizing adjusts the letter to hold volume constant.
Should I sister size up or down?
If the band rides up, sister down a band (tighter). If the band digs in, sister up a band (looser). Keep the cup volume matched using the calculator.
Are sister sizes the same across brands?
Roughly, but brands cut differently, especially beyond DD cups. Use sister sizes as a starting point and fit check each brand.
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