Shopping for clothes is generally a pretty straightforward experience. Sizes become larger as they advance, and that’s that. Bra sizes, on the other hand, are not quite as simple. You have to consider factors such as your unique breast shape, your band size, and your cup size — all of which rarely matches neatly with a single number and letter. This is where sister sizes come in.

What Are Sister Sizes?

Sister sizes are groups of bra-size equivalents that are related by cup volume. For example, a 36F (DDD) cup belongs to the same family as a 38E, a 40D, and a 42C. Stick with us — we’ll explain. But just know that your perfect fit may still be out there waiting for you. And we’re going to help you discover yours today.

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. Wrong.

Bra sizes are more like ratios. The volume of a cup size (or how much breast tissue it can hold) varies with the band size. Let us break it down for you.

The Importance of Knowing Your Bra Sister Size

If you’ve ever struggled to find a bra that fits your unique shape, you’re not alone. It’s no easy feat, and even less so when you’re unfamiliar with your sister sizes. Remember: it’s not you, it’s your bra. Identifying your sister size can be especially helpful if you find that neither your cup or band fit properly.

How to Calculate Your Sister Size

Go up a band size (if your band is too tight), but go down a cup size. The same rule applies in the reverse if your band is too loose: go down a band size, but go up a cup size. We promise It only sounds more complicated than it is, so we created a handy sister size calculator for you to reference.

Find your current bra 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 30E, 32D, 36B, and 38A. Each row is a family of sister sizes.

Let that sink in for a moment: an A cup can also be an E cup. Wild, right?

What’s amazing about sister sizes is that although they can be counterintuitive, they’re key to finding a great-fitting bra. For example, if you’re wearing a 34C and the cups fit, but the band is small, you might think “I’ll just go up a band size and try a 36C.” However, sizing up that way will likely result in cups that are too big because a 36C’s sister size is a 34D. The next sister size to a 34C is a 36B, which is the new size to try instead. The cups will still be the same size, but the band will be roomier.

Here’s a breakdown of what to do in a few specific scenarios:

Your band fits, but your cups don’t: If the cups feel small, go up a cup size. If the cups feel big, go down a cup size. (Ex. If the 32F feels small in the cups, go up to a 32G. If the 32F feels big in the cups, go down to a 32E1/2).

Your cups fit, but your band doesn’t: Adjust both your band and cup size. Remember: if you go up a band size, you need to go down a cup size, and vice versa. (Ex: A 34C cup is the same as a 36B. A 40E(DD) cup is the same as a 42D.)

Neither your cup or band fit: Go down a band size if they’re both too big, or go up a band size if they’re both too small. Don’t worry about changing your cup size — it’ll natural adjust with your band! (Ex: If a 32D is too small, try a 34D.

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.

With additional reporting by Cathy Bishop and Nicole Yi.