What Is a Sister Size in a Bra?

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Young girls then undergo puberty and become grown adults. Even as grown women, we still do encounter some difficulties with choosing the correct bra size or fit that is best suited for our bodies. This leads us to the question, what is a sister size in a bra?

What Is a Sister Size in a Bra?

A sister size in a bra is an alternative bra size having the same (or very similar) cup volume with a different band size or cup letter. You may find that you are one size in a certain brand, and a different size in another. By knowing your bra sister sizes, this can help you more easily determine your bra size when buying bras from different brands.

A woman in a pink push-up bra is fixing the strings wearing black leggings

Understanding Your True Bra Size

Sister sizing is a game-changer when it comes to looking for an alternative fit, especially because 70 to 100% of women wear the wrong size of bra.

This is also definitely useful since different bra brands tend to have different measurements. Moreover, it is definitely useful especially when your true bra size got sold out or when you’re looking for a looser or tighter fit.

Before trying to know your sister size, it is best to measure your band and cup size first. Let’s learn the different numbers and letters associated with bra sizes.

Every bra size is composed of 2 components: an even number component and an accompanying letter component.

  • Even number component: Indicates your band size that is measured around your torso, directly underneath the breasts (eg. 34, 36,38)
  • Letter component: Indicates your cup size that is determined by using a measuring tape and measuring the fullest part of your breasts (eg. AAA, A, B, C)

Understanding Your Sister Size

Sister sizes are alternatives, having the same cup capacity, with a difference in band size or cup letter. It pretty much depends on the brand that you’re dealing with. There’s no need to worry, though, as major brands are equipped with prepared size charts helping you determine your “correct” sister size.

Sister sizing is particularly useful in scenarios wherein you are having problems with the looseness or the tightness of the band fit of your well-fitted bra cup. You even have the option to size up or down depending on your preference and comfort.

Examples of Your Preferred Bra Fit

Here are some examples that can help you understand the concept of sister sizing and decide on what works for you. In the coming set of examples, the bra size to be used as a starting reference is 34B.

  • Tighter Fit: If you want to get a tighter fit around the band, then go and try a 32C bra size since it has a smaller band circumference while still providing the same cup volume.
  • Looser fit: If you want a looser band fit, you can go one size up with the band size and a cup size lower. Your looser fit size is 36A.

Here’s a formula you can use to determine your sister size based on if you want a tighter or looser fit from your true (or standard) size. We’ll use 34B as the true size in the examples below:

  • Tighter Fit: 34B – 1 band size + 1 cup size = 32C sister size
  • Looser Fit: 34B + 1 band size – 1 cup size = 36A sister size

Notice the pattern? So if you plan on going for a tighter fit, get a band size lower and compensate for it by sizing up for the cup size. As for a looser or more comfortable fit, size up with the band then size down with your cup size so that the cups will still be a good fit or support for you.

When to Size up or Down

To size up or down will be up to your preference. It is very important that you know your true bra size. If you still don’t know your true fit, you can get it correctly by having a bra fitting done. From there, here are some tips that could help you in your sister sizing journey.

Cup Fit

If the fit of the cups are supportive, with no gaps or boob spilling, then your problem might be the band fit and you can definitely try sister sizing. If gaps or boob spilling are present, it is best to get a proper bra fitting done.

Fastening Hook Preference

Here are some things to consider with the fastening hook:

  • Tightest hook preference: It is best to try going a size down. Again, going a band size down means you have to go 1 cup size up. Example, from your true fit size of 34B, your sister size is 32 C.
  • Middle hook preference: You can either go a size up or down, depending on the comfort or fit level you’re aiming for. Keep in mind the cup support must be okay for you as well.
  • Farthest hook preference: It is best to try going a size up. Going a band size up means you have to go one  cup size down.  Example, from your true fit size of 34B, your alternative size is 36 A.
A woman in a pink push-up bra is fixing the strings wearing black leggings

It is again very important to note that sizes may vary from brand to another. Knowing sister sizing will definitely help us women navigate and explore our choices and to not be limited in our journey of finding the “perfect” bra fit that will get the job done. It provides us with more options that we never knew we had before.

Related Questions

Will My Sister Size Fit Me as Well as My Exact Size?

No, your sister size may not fit you like your exact size, although you can achieve the same comfort of your true size, with some adjustments like tightening or loosening the straps. Another great thing about this is that this may actually help you realize that you are wearing the wrong size all along.

Do Sister Sizes Work All the Time?

Sister sizes don’t work all the time. Different brands may have slight inconsistencies with the styles and capacity of the bra, so it is still important to try it on before checking it out.

Bra Size Measurement & Cup Size | A, B, C, D, DD Bra Sizes AND Extra Large Breasts

Final Words

When in doubt, shop clerks are very much helpful and are very accommodating with regard to our bra queries. Never be embarrassed to need and ask for help.  Major bra brands also have prepared charts for sister sizing, making our lives easier.  Enjoy and happy bra shopping!


Hi there, I’m Allena, Founder and Editor of The Better Fit. I’ve always had a passion for finding ways to help women improve various areas of their lives. I have researched and worked to create this website so women would have a place to come and be informed about an article of clothing they literally wear every day.