Sister Sizing Explained: How Band and Cup Changes Relate
Learn the common sister-size rule, why it is useful for fit experiments, and why sister sizes are not physically identical across every bra.
The Common Sister-Size Rule
If a band is too loose but the cup appears suitable, a common fit experiment is to move down one band and up one cup. If the band is too firm while the cup appears suitable, the reverse move can be tried.
- 36C → 34D
- 34D → 36C
These relationships are most useful within the same sizing system. Cross-system conversions require a separate size chart.
Why Sister Sizes Are Not Identical
Moving across bands changes more than the label. The band circumference, wire geometry, cup proportions and strap placement can all change. Therefore a sister size should be treated as an adjacent alternative to test, not as a mathematically identical physical cup.
Use Fit Checks With Sister Sizing
University of Portsmouth research recommends checking the underband, cups, underwire, centre front and straps rather than relying solely on a calculated size. citeturn1search1
Frequently Asked Questions
Answers about sizing methods, measurements, and fit considerations.
What is the sister-sizing rule?
The common rule is to move down one band and up one cup, or up one band and down one cup. This aims to keep a similar cup capacity within the same sizing system.
Are sister sizes exactly identical?
No. They are a useful grading relationship, not a promise of identical physical cups. Wire width, cup depth, band length, strap position and construction change with size.
Sources & Standards
- White & Scurr (2012), Ergonomics, DOI 10.1080/00140139.2011.647096