Bra Sister Sizes
Move one band step and one cup step in the opposite direction when the volume feels close but the band does not.
Sister-size rule
Move up one band and down one cup, or move down one band and up one cup, to keep roughly similar cup volume.
Example: a common sister-size relationship is 34C ↔ 36B ↔ 32D. Letter sequences differ between sizing systems, so use the rule within the same system.
When sister sizing is useful
Sister sizing is most helpful when the cup volume feels close but the band is clearly too tight or too loose. Moving one band step requires an opposite cup-letter adjustment to preserve roughly similar volume.
It is not a universal fix. A different wire width, cup depth, or bra shape may be needed even when the nominal volume is similar.
How this guidance is checked
These sources support the measurement method and help check real-world variation. They are not presented as one universal brand-size table.
- Bra Fit Calculator — Wacoal. Bra measurement cross-check using underbust and fullest-bust measurements and brand-specific fitting logic.
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