May 4, 2026
Sister sizing in bras, explained simply
What the sister-size matrix means, why it matters, and how it opens options in boutique ranges — in plain language.
Sister sizing is one of those concepts that sounds technical but resolves into something quite simple: two bra sizes share the same cup volume when one goes up a band size and down a cup letter, or vice versa. A 34C and a 32D hold the same volume of breast tissue. A 36B and a 34C are sisters too.
This matters practically for two reasons: it tells you where to look when your exact size is not available, and it explains why some bras fit better at a sister size than at the measured size.
The matrix, explained
The cup letter in a bra size describes the difference in inches between the underbust and over-bust measurements — not an absolute volume. A C cup on a 32 band contains less volume than a C cup on a 36 band, because the cup is proportioned to the band.
The sister-size relationship follows from this: if you go up one band size and down one cup letter, the cup volume stays constant but the cup is proportioned for a longer underwire circumference. If you go down one band and up one cup letter, the cup volume stays constant with a shorter underwire.
Some examples:
- 32D = 34C = 36B (same cup volume, different band circumference)
- 34DD = 36D = 38C (same cup volume, different band circumference)
- 30G = 32F = 34E (same cup volume, different band circumference)
The sisters extend in both directions — smaller bands and larger cups, larger bands and smaller cups — for as many steps as the size range allows.
When sister sizing is useful
When your size is out of stock. Boutique ranges are often produced in limited runs, and popular sizes sell through. If your measured size is unavailable, the sister size in the same style will fit the cup volume correctly, with the band requiring adjustment via the hook-and-eye positions.
When the band fits but the cup does not. If the band measurement is correct but the cup gaps, you may need less projection — try the sister size down one cup (and up one band). If the cups overflow while the band is correct, try the sister size up one cup (and down one band).
When the brand runs small or large in the band. Different brands cut their bands with different amounts of negative ease. In a brand that runs tight in the band, the sister size with one step up in band is often more comfortable without changing the cup volume.
The limits of sister sizing
Sister sizing adjusts volume and band circumference together. It does not adjust the underwire circumference, the cup height, or the strap position. A bra that is a correct sister size in volume may still not work if the underwire is too wide or narrow for your breast root, or if the cup height is wrong for your torso. Sister sizing is a useful first step, not a guaranteed fit.
For the full fitting picture, the bra fit guide and the guide to what a correctly fitting bra feels like cover the complete fit assessment.