Sister Sizes: The Bra Secret Every Woman Should Know

A few weeks ago, I did an informal poll of my women friends. It went something like this:

Me: Quick, what’s your bra size?

Friend: 36C, why?

Me: What’s your sister size?

Friend: My sister’s size? Weird question. I think she’s a 34B. Can you pass the guacamole?

I was asking about sister sizes, not what a sibling’s bra size is, but I understand the confusion. It turns out that almost no one I asked had ever heard of sister sizes. And it’s a shame, because sister sizes might just forever change the way you wear bras.

Chances are, you learned somewhere along the line that you had a bra size and if it sort of fit, that was basically the end of the story. You may have also thought that unless one’s breasts actually changed size, there was just no way someone who was, for example, a B cup could also be a D cup. Just not possible.

It turns out that’s totally wrong! Bra sizes aren’t like clothing sizes (where a 12 is larger than an 8 and that’s that)—they’re actually more like ratios. The volume of a cup size (how much breast tissue it can hold) varies with the band size. Go up a band size and you need to go down a cup size for the equivalent cup volume. That’s how you calculate your sister size. It sounds confusing, so we created a handy chart for you to reference. Find your size, and your sister sizes are the ones listed in the same row (going across). So, for example, if you’re a 34C, your sister sizes are 36B and 32D, and 38A and 30E:

Let that sink in for a moment: an A cup can also be an E cup. Wild, no?

What’s amazing about sister sizes is that although they can be counterintuitive, they’re key to finding a great-fitting bra. If you’re wearing a 34C and the cups fit but the band is too small, you might think “I’m a C cup, so I’ll try a 36C.” But sizing up that way will likely result in cups that are too big, because a 36C’s sister size is a 34D. You should try a 36B instead. The cups will still be the same size, but the band will be roomier.

And sister sizes work with our signature half-cup sizes, too. If you’re a 34B½, you’re also a 36A½, and a 32C½. Because each of those sizes have equivalent cups, the bra you choose depends on which band size feels best to you.

Now that you know about sister sizes, you’ll be equipped to choose bras that fit you more precisely. Our goal at ThirdLove is to help women find their perfect fit, so don’t keep sister sizes a secret. Tell a friend, coworker, or—of course—your sister.