Bra Sister Sizes Explained: Similar Cup Volume, Different Band

A cup letter is relative to its band

A B cup is not one fixed volume independent of band size. Wacoal’s b.tempt’d guide uses 34B, 32C and 36A as a sister-size example: moving the band changes which cup letter gives a similar volume. Keep the band and cup together when describing a bra size.

Change one band step and one cup step

For a nearby sister size, move up one band step and down one cup step, or down one band step and up one cup step. In the stated example, 34B to 36A loosens the band while keeping a similar cup volume. Stay within the same brand’s cup sequence when choosing the step.

Use sister sizing when the cups fit

If cups already contain breast tissue comfortably but the band is too tight or loose, a sister size can be a useful comparison. If cups spill, gape or sit on tissue, changing band alone is not necessarily the solution. Check the full fit and consider a different cup shape.

Do not convert across brands by letter alone

US, UK and European cup naming diverges, particularly after D. A sister-size step means the next cup in the relevant chart, not necessarily the next alphabetic letter. Construction and stretch also change fit, so an equivalent label is still a starting point.

Before you order

1. Identify the brand and cup sequence of the current bra.
2. Check whether the problem is band fit or cup fit.
3. Try one nearby sister size rather than a large band jump.
4. Confirm level band, comfortable support and contained tissue.