“Sister sizing” is a term used to describe the same cup sizes (letter) when the band size (number) changes. Usually calculating your “sister size” is helpful when solving common bra fit issues, such as changing band size (number).
Sister sizes have the same cup size as each other, although represented by a
different letter depending on the band size.
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1 SISTER SIZE DOWN = one band size smaller than your current band
and one letter larger than your current cup ie. 34C = 32D
As your band size gets smaller, your cup size gets bigger.
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1 SISTER SIZE UP = one actual band size larger than your current
band size and one letter smaller than your current cup ie. 34C = 36B
As your band size gets larger your cup size gets smaller
Horizontally this chart represents bras with the same cup size as the band size changes, although the letter differs the cup size is the same.
Start with your usual size:
Move left to see your sister size down – this is helpful if your cup feels good but your band is too loose.
Move right to see your sister size up – this is helpful if your cup feels good but your band is too tight.