Sister Sizing: Our Favorite Bra Shopping Hack

Imagine two different bras: one is size 38A, and the other is size 32DD. Which bra has the bigger cup?

You probably imagined the bras like this.

Bra sister sizing example

In fact, it’s more like this.

Sister sizing example

How is that possible?

Bra sizes 38A and 32DD actually hold the same volume of breast tissue.

This is due to our favorite bra-shopping hack called sister sizing.

What is sister sizing?

Remember that a bra cup size is always relative to its band size, calculated as the difference between the bust circumference and the torso circumference. Therefore, as the band size increases, the same cup size will hold more breast volume. So, the cup size for 36B is larger than the cup size for 32B, even though they are both sized B. By the same logic, bra size 36B holds the same volume of breast tissue as size 34C. Check out the diagram below to get a feel for how this works.

sister sizing visual example

Many women have hacked this system to help with bra shopping. You can too! If you find a bra that you like, but your typical size doesn’t fit correctly, you can try a bra in a sister size.

How do I find my sister size?

You may have figured out that rather than a static figure, your bra size actually falls somewhere along a range of sizes that have equivalent volumes. To find your sister sizes, take your regular bra size and go up a size in one factor and down a size in the other factor. It sounds more complicated than it is.

Here’s an example: If you are a 34B, one of your sister sizes would be 32C (down a band size, and up a cup size). As you can imagine, there are a lot of sister sizes for each bra size.

Below is a chart to help you sort it out. Find your bra size, and every size in the same row (going across) are your sister sizes. If your size isn’t on the chart, use the international size conversion table below.

sister sizing chart
International bra size conversion chart

How do I use sister sizing to find a bra that fits?

To start, try on a bra that is your typical size. Does it fit correctly? Here’s how to tell.

If the band fits but there are gaps between your cups and breasts:

Choose a sister size with a larger band size and smaller cup size (36B → 36A).

If the band fits but the cups cut into your breast tissue:

Keep the same band size but go up in cup size (36B → 36C).

If the cups fit but the band is too tight:

Choose a sister size with a larger band size and smaller cup size (36B → 38A).

If the cups fit but the band is too loose:

Try a sister size with a smaller band and larger cup (36B → 34C).

If neither the cups nor the band fit:

If the bra doesn’t fit right but it’s close, opt for a bra with a different band size (36B → 38B). Changing the band size means that both the band size and the cup size will change.

We hope our tips on sister sizing makes finding the right bra a little less intimidating.

When bra shopping, keep in mind that if a bra doesn’t fit no matter how you tweak with the sizing, it could be an issue of the cup shape, rather than the size itself. Check out our Fit Quiz to learn more about your breast shapes and which bras are best for you!

Have you heard about sister sizing before? Let us know in the comments!

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