Bra Sister Sizes: Cup Size Charts & Secrets

Posted by Alexis Canary on

Bra Sister Sizes: Cup Size Charts & Secrets

Bras have a secret, and it’s not Victoria’s.

Most bra sizes have a companion “sister size”—a size that functions similarly and that can sometimes provide a better fit depending on a woman’s bust shape, shoulders, and other factors. There are perennial news stories stating that most women wear the wrong bra size. But the reality is that thanks to variations between brands and even individual styles, there may be more than one size that “fits.”

The key is finding the one that’s most comfortable. In this short guide, we’ll explain sister sizes and the steps on how to find the right bra for your body.

Sister Sizes 101

It’s common knowledge that your bra size is supposed to be based on two measurements—your band circumference and your cup. The only problem? Cup size isn’t a static measurement.

  • Cup size refers to the difference in circumference between your underbust measurement and the measurement at the fullest part of your bust.

  • The cups on a 32A bra have a different volume than the cups on a 34A bra. In fact, they’re more similar to a 34AA!

If your bra is ill-fitting, it can be tempting to move up (or down) by underbust measurement or by cup size. But adjusting only one of these measurements could present a new gamut of problems.

Instead of shifting in one direction, try shifting in two! 

There are many sister size charts outs there, and we’re confident they can help. If you wear a size on the left and it’s not quite the perfect fit, try shifting to the right.























Bra Comfort 101

Wait, you might be asking. Do I really need a different bra size? Aren’t bras supposed to be uncomfortable?

Our answer in this case is a resounding no!

If you find yourself constantly adjusting your straps, struggling with pinching sensations, or bulging out from beneath your t-shirts, it’s totally possible that incorrect bra size is to blame.

Let’s take a look at the three criteria for the perfect fit.

Your Band Size Can Be the Basis For Fit

Because band size is the most objective measurement of fit, it’s the best place to start. To measure your band size:

  • Use a tape measure to take the circumference of the area just under your bust. Be sure to hold it level.

  • Keep in mind that bands have eyelets that allow for 2” or more of adjustment. If you fall between measurements  (say, at 31), you could try a 30 or 32 band width.

Figuring out which will be more comfortable depends on your breast shape. If you’re already wearing a 32, for example, look for signs that it’s too big.

  • Do your bra straps droop down your shoulders at all hours of the day? If you have narrow or sloped shoulders, it’s possible that a racerback style bra could help. But the need to constantly adjust is also an indication of a loose-fitting band.

  • If you can easily pull your band away from your body, that’s another sign that it may be advantageous to go down a band size.

Consider the Cup

When you got your first bra fitting, someone measured your underbust, followed by the fullest part of your chest, and then made a simple calculation:

  • Underbust circumference (minus) bust circumference = cup size

This relative measurement leads to the following standard cup sizes:


Less  than 1”











(and so on).

Of course, this doesn’t take into account “in-between” sizes. To remedy this problem, some bra brands have expanded their size offerings.

But beyond volume and size, bra cups are different shapes. From sewn triangular cups to push-up cups to demi-bras and beyond, bras are built to accommodate a variety of breast shapes. They accomplish this goal with varying success.

Which size is right for you? It can be easier to figure out which is wrong.

  • Your cup may be too small if there’s a visible gap between your breast and the top of the bra or you constantly find yourself readjusting your straps to get more support.

  • It may be too big if your breast tissue feels cut into or constrained by your bra. In t-shirts, you might have a bump or “double boob” above the lip of the cup.

  • These are sure signs that it’s time to experiment with sister sizes.

    How Bra Sister Sizes Can Help

    But once you add cup size to the mix, there can be even more uncertainty. For example, If your band size is 32.5” and your bust size is 34.5”, you could potentially try out all of the following options:

    • 38AAA
    • 38AA
    • 36AAA
    • 36AA

    Which will fit best? It all depends on the unique shape of your body, including your rib cage, shoulders, and bust. As we’ve noted, sister sizes bear a family resemblance.

    If you think you’ve picked the wrong size, refer back to our handy bra sister size chart US. If you’re wearing one of the current sizes and it feels too roomy, shift to the right. If it feels too snug, shift to the left.

    Lulalu: Bra Shopping for Petite Frames, Demystified

    At Lulalu, we know that women with smaller frames can face even more challenges when browsing the bra aisle. From noodly straps to gaping cups and beyond, it’s hard to find the perfect fit.

    That’s why we’ve made it our mission to help you find the right bra size. Once you’ve taken our quiz, shop our selection of stylish petite lingerie that’s sure to hug your frame while keeping you comfortable.

    Bras aren’t supposed to hurt! If yours does, let us introduce you to a new world of comfort.


    New York Times. Are 8 Out of 10 Women Really Wearing the Wrong Bra Size.

    Cosmopolitan. How to measure your bra size in 4 easy steps.


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