Not All D Cups Are the Same

One of the biggest misconceptions in bra fitting is that the cup letter tells you everything.

It doesn’t.

Woman wearing a D cup bra.

A 30D, 34D and 38D are completely different sizes. The D tells you the relationship between the cup and the band. It doesn’t tell you how large the breast itself is.

This is where sister sizing comes in. Sister sizes have roughly the same cup volume but different band sizes. For example, 30D, 32C, 34B and 36A are sister sizes. As the band gets larger, you go down a cup; as the band gets smaller, you go up a cup.

But sister sizes aren't interchangeable. The proportions and fit change with the band, so you can't simply swap between them and expect the same bra to fit in exactly the same way.

This is one of the reasons so many women are surprised during a professional fitting. You might discover that the cup letter you thought you needed has very little to do with the actual size and shape of your breasts.

The letter is only part of the story. The band, the cup and the shape all have to work together.