Sister Sizing Explained: Why Two Different Bra Sizes Can Fit the Same

If you have ever heard that a 34B and a 32C can fit almost identically, it probably sounded confusing at first. This concept, known as sister sizing, is actually one of the most useful things to understand when shopping for bras, especially when your exact size is out of stock or when a specific style runs slightly differently than expected.

What Sister Sizing Actually Means

Sister sizes are different band and cup combinations that hold roughly the same cup volume. As you move one band size up, the cup volume increases, so to keep the same actual cup volume, you move one cup letter down. This is why a 34B and a 32C, or a 36B and a 38A, are considered sister sizes: the total volume in the cup stays similar even though the labeled size looks completely different.

Why This Happens

Cup size is not a fixed, standalone measurement, it is relative to the band size. A C cup on a 32 band holds a different actual volume than a C cup on a 38 band, even though both are labeled C. Sister sizing simply reflects this relationship: change the band, and you have to adjust the cup letter to keep the same real volume.

When Sister Sizing Is Useful

This concept is especially helpful in a few common situations. If your usual size is out of stock, trying the sister size in a slightly different band can sometimes give you a similar overall fit while you wait for restock. If a specific bra style runs a little snug or loose in the band compared to your usual brand, shifting to the sister size can compensate without needing to completely rethink your size. And if you are between two sizes and unsure which to order, knowing the sister size relationship helps you make a more informed choice rather than guessing.

What Sister Sizing Does Not Fix

It is important to understand that sister sizing keeps cup volume similar, but the band fit itself will still be different. A 34B and a 32C hold similar volume in the cup, but the 32 band will fit more snugly around your ribcage than the 34 band. If your main fit issue is specifically with the band feeling too tight or too loose, sister sizing within the same volume will not solve that, since you are intentionally changing the band as part of the swap.

A Simple Way to Remember It

Moving up one band size means moving down one cup letter to stay at the same volume, and moving down one band size means moving up one cup letter. This pattern holds consistently across most sizing systems, which makes it a genuinely reliable rule of thumb once you have it memorized.

Combine This With Your Actual Measurements

Sister sizing is a helpful tool once you already know your baseline size, but it works best alongside actual measuring rather than as a replacement for it. If you have not measured yourself recently, start with our bra size calculator to get your baseline size, then use sister sizing as a flexible reference point from there.

Shop a Range of Sizes at Nudra

Nudra's bras span a wide size range across bands 32 to 42, giving you room to try sister sizes within the same style if your first choice does not feel exactly right, with Cash on Delivery across Pakistan for easy exchanges.