Bra Sister Sizes Explained
If the cups fit but the band doesn't, you don't need a whole new size — you need a sister size.
Cup volume stays the same when you trade band for cup: down a band → up a cup (34C → 32D), up a band → down a cup (34C → 36B).
A bra cup isn't an absolute size — it's relative to the band. A "C" on a 32 band is smaller than a "C" on a 38 band, because the cup is measured as the difference between your bust and band. So if you keep that difference constant and shift the band up or down, the cups hold the same volume. Those combinations are sister sizes. They're the fix when the cups are right but the band rides up (too big) or digs in (too small).
Sister size chart
Each row holds about the same cup volume; the band tightens left → right is loosened (smaller band number = tighter band).
FAQ
What is a sister size?
A sister size is a different band + cup combination that holds the same cup volume. Going down one band and up one cup (e.g. 34C → 32D) keeps the cups the same size while making the band tighter.
When should I try a sister size?
When the cups fit but the band feels too tight or too loose. Loosen the band by going up a band and down a cup; tighten it by going down a band and up a cup.
Does a 34C fit like a 32D?
The cups hold roughly the same volume, but the band is tighter on a 32D. They are sister sizes, not identical — the band length and feel differ.
How do I find my sister size?
Up a band → down a cup; down a band → up a cup. Keep stepping one band at a time. Our calculator gives your starting size; then adjust by band feel.
Based on the standard bra-fitting difference method (cup = bust − band). General reference; fit varies by brand and style.