Bra sister size calculator
If the band feels tight or loose, a sister size keeps the cup volume while changing the band.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter sizing system, band and cup.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Smaller sister = band one step down (2 in or 5 cm) and cup one letter up. Larger sister = band one step up and cup one letter down. Two steps repeat that twice.
Example
Inputs: Sizing system = UK, Band = 34, Cup = DD. Result: Sister size, smaller band: 32E; Sister size, larger band: 36D; Two steps, smaller band: 30F.
Limits and caveats
- Cup volume is close, not identical, and shape varies by brand.
- Going more than two steps away changes the fit a lot.
Questions about this tool
What is a sister size?
A size with a different band and adjusted cup so the cup volume stays about the same, so a 34D cup is close to a 32DD cup.
When should I use a sister size?
When the cup fits but the band is tight or loose, or when your size is out of stock.
Why does it say not available?
The resulting band or cup is off the chart, such as a band under 28 or a cup below AA.