Not sure of your exact size yet? Use the bra size calculator first.
A schematic fit diagram, scaled to your cup size.
How the chain works
Every time you move one band size up, you gain roughly the same amount of extra cup room that moving one cup letter down takes away — so the actual volume held stays close to constant. That's the entire trick behind sister sizing.
band −2, cup letter +1 → same volume
A 34D, for example, sits in the same volume family as 32DD and 36C. None of these are "more accurate" than the others — they're different ways of packaging the same volume, and which one actually fits best comes down to how your ribcage and band preference line up.
When to use it
Reading your own fit signals
Go up a band, down a cup
If the cups look right but the band digs in or leaves marks, move one band size up and one cup letter down.
Go down a band, up a cup
If the band rides up your back even on the tightest hook, move one band size down and one cup letter up.
Try the next size over
Sister sizes are the fastest workaround when your exact size isn't available in the style you want.
Different cut, same chain
Since brands cut differently, trying a sister size is often faster than guessing at a completely different size.
Questions
Sister size FAQs
A different band-and-cup combination that holds roughly the same cup volume as your current size. Moving one band up and one cup letter down (or the reverse) keeps that volume nearly constant.
If your band feels tight but the cups fit, go up a band and down a cup. If your band feels loose but the cups fit, go down a band and up a cup.
Not directly — sister sizes preserve cup volume rather than change it. They solve band comfort, not gaping or spillage in the cup itself.
Most people stay within one or two steps before shape starts to change meaningfully, even though the volume keeps tracking closely on paper.
The one-band-up-one-cup-down logic holds in every inch-based system (US, UK, Canada, Australia). Metric systems follow the same idea once converted to their own band numbers.
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