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Understanding Bra Sizing: The Mathematics of Bands, Cups & Sister Sizes
Just ask and get Editorial Team(Apparel Engineering & Fit)
Updated 2026-09-20
6 min read
Key Takeaway:Demystifying bra band and cup measurement math, sister sizing mechanics, and international size conversions.
Cup Letters Are Ratios, Not Fixed Volumes
A common misconception is that a "D cup" represents a fixed volume. In reality, cup volume scales directly with band size: a 30D cup holds substantially less breast volume than a 38D cup.
Frequently Asked Questions
Where should most bra support come from?
Approximately 85% to 90% of bust support should come from a snug, properly anchored ribcage band, not from shoulder straps.