Sister Size Calculator
Sister sizes are nearby sizes with a similar cup-volume relationship but different band and cup labels. They can help when your current band or cup does not fit comfortably.
How sister sizing works
When you move down one band, you generally move up one cup letter to keep a similar cup-volume relationship. When you move up one band, you generally move down one cup letter. For example, a UK 34C has nearby sister sizes 32D and 36B.
Smaller band
If the band of your current bra feels loose or rides up, one nearby option is a smaller band with the next cup letter up. Check that the cups still fit correctly.
Larger band
If the band feels too firm, one nearby option is a larger band with the next cup letter down. This changes the fit around the ribcage, so it is not identical to your original size.
Example
For 32DD, the nearby UK sister sizes are 30E and 34D. The cup label changes because the band circumference changes.
When should you try a sister size?
- The cups appear to fit but the band is clearly too tight.
- The band rides up while the cups appear appropriate.
- You are between available sizes in a particular brand.
- You want to compare adjacent sizes before deciding whether a different cup shape is needed.
When sister sizing is not the answer
If the cup is cutting into breast tissue, gaping badly, or the underwire shape is wrong, changing the band alone may not solve the problem. A different cup size or bra shape may be more appropriate. Use the Bra Fit Check to diagnose the issue.