How Bra Cup Sizes Work: Relative Volume Capacity Explained
By BraSizeChecker Editorial Team•
One of the biggest misconceptions in apparel sizing is that a "D cup" is a specific size. In reality, cup letters represent a ratio, not a fixed volume.
1. Cup Volume Scaled to Band Size
Because bra cups are scaled proportionally to the band width, a 30D cup holds significantly less physical volume than a 38D cup.
CUP VOLUME COMPARISON:
• 30D Cup Volume = Approx. 390 cm³
• 34D Cup Volume = Approx. 590 cm³
• 38D Cup Volume = Approx. 850 cm³
2. Equal Volume Equivalents (Sister Sizes)
Because cup volume is relative, moving up a band size while moving down a cup letter preserves the exact same cup volume capacity:
32D = 34C = 36B = 38A (Equal Cup Volume Capacity)
Calculate your volume equivalents with our Sister Size Calculator.