Bra Sister Size Calculator
Select your current UK bra size to instantly see all sister sizes — bras with the same cup volume but a different band.
What are sister sizes?
Sister sizes share the same cup volume but have different band lengths. Going up a band size means going down a cup letter — and vice versa. This is useful when your preferred size is out of stock, or you need a different fit around the ribcage.
Frequently Asked Questions About Sister Sizes
What are bra sister sizes?+
Bra sister sizes are groups of bra sizes that share the exact same cup volume, even though their band numbers and cup letters are different. For example, a 34C and a 36B have the same cup volume, but the 36B has a wider band.
How does sister sizing work in the UK?+
The rule is: if you go down one band size (e.g. from 34 to 32), you must go up one cup letter (e.g. from C to D) to maintain the same cup volume. Conversely, if you go up one band size (from 34 to 36), you must go down one cup letter (from C to B).
When should I use a sister size?+
You should use a sister size if your calculated size is out of stock, or if your bra band feels slightly too tight or too loose but the cup volume fits you perfectly. For example, if your 34C cup fits but the band is too snug, try a 36B.
Is a 34D cup the same size as a 36D?+
No. A 36D is one cup volume larger than a 34D. Cup sizes are not absolute; they are relative to the band size. A D cup on a 36 band holds more volume than a D cup on a 34 band.
How do I find my sister sizes?+
Select your standard UK bra size in our sister size calculator above. It will instantly list your closest alternative sizes with the same cup volume (both a band size up and a band size down).
Expert Verified Sister Sizing
All alternative fitting logic is audited by professional bra fitters.
Eleanor Vance
Certified Bra Fitter & Lingerie Consultant
Over 12 years of professional fitting experience at leading UK lingerie retailers, including Bravissimo and Marks & Spencer. Specially certified in maternity, teen first-fit, and post-surgery supportive consultations.
Sizing Math & Rounding Logic
Our calculator implements the industry-standard **modern fitting method** rather than the obsolete "plus-4" retail formula. Band sizes are derived directly from snug ribcage measurements in inches (rounded to the nearest even number). Cup sizes are calculated using the precise 1-inch difference increments between fuller bust and underbust measurements (e.g. 4" = D, 5" = DD, 6" = E).
Sourced Claims & Studies
Medical and biomechanical research shows that up to **80% of UK women wear the wrong bra size**. According to breast health clinical research from the **University of Portsmouth (Department of Sport and Exercise Science)**, properly fitted encapsulation support reduces vertical breast movement by up to 59%, significantly preventing exercise-induced breast discomfort and cooper's ligament stretching.