Equivalent cup volume
Bra Sister Size Calculator for a Firmer or Roomier Band
Move one band size while changing one cup index in the opposite direction to preserve approximately the same cup volume.
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Bra sister size calculator for a better band fit
Sister sizes keep similar cup volume, but cup shape, wire width and support can still feel different. UK sizing uses double letters such as FF, GG and HH after F.
What bra sister sizes mean for band and cup volume
Sister sizes pair a smaller band with a larger cup letter, or a larger band with a smaller cup letter. The cup volume stays similar even though the full size label changes.
How to calculate sister sizes for a 34DD bra
In UK sizing, the firmer sister size of 34DD is 32E and the roomier sister size is 36D. This can help when the cups fit but the band tension is clearly wrong.
When a bra sister size will not solve the fit problem
Sister sizing does not correct cups that are too shallow, too projected, too tall or too narrow. A two-band jump also changes support and proportions enough that a different style may be more useful.