Bra Sister Sizes: Here's What You Need To Know

A woman laughs and puts her head in the lap of another woman as they sit on a couch, both in racerback bralettes.

We’ve all been there: you’ve carefully measured yourself so you know your bra size, and you’ve researched the best bra for you. And then… your size is out of stock, or doesn’t exist in the brand, or just doesn’t fit like you think it should. And while this may seem like you’re just outta luck and you need to (ugh) keep bra shopping, there is another option: bra sister sizes!

Yep, one of the best OG life hacks for someone with breasts is to know their bra sister sizes. Of course, sister bra sizes aren’t exactly as easy as looking at an underwear size chart and picking what seems like the equivalent size and hoping for the best.

Today I’ll tell you everything you need to know about bra sister sizes, like what they are, how to measure, and how to double check the fit of your sister bra sizes. Once you know your sister sizes, you’ll have more options when finding that perfect bra, perhaps when you’re shopping for a racerback bralette or triangle bralette.


Two women, one with a black afro and one with long dark hair, sit on a white couch, modeling black underwear.

Bra sister sizes are just like they sound: they are the sizes that are essentially equivalent to your bra size, but with different cup sizes and band sizes. However, even though they aren’t the same cup or band sizes, the cup volume stays the same. This stays the same because bra sizes are more like ratios: so, if you increase your cup size, you decrease your band size, to keep the ratio of tissue the same.

Why do sister sizes matter, you may ask? Since most women, around 80%, wear the wrong bra size, I have a feeling you’ll already know the answer to that question: bra sizing is tricky business! Even if you measure yourself properly, standard bra sizes don’t account for half-sizes, different breast shapes, and so much more. Knowing your bra size, and your sister sizes, can help you find a bra that really and truly fits. ‘Cause baby, you deserve that!


A woman with black curly hair stares ahead as her collarbone is measured with white measuring tape.

Before you know what your sister bra sizes are, you need to know your main size! To do so, grab some measuring tape and lose that shirt! Measure your rib cage below your breasts. Add four for even numbers, and five for odd numbers. Then measure your breasts at their fullest. Subtract this from your band size number to get your cup size.

So, let’s say your bust size is 40, and your band size is 38—there is a two-inch difference between your bust size and your band size, which correlates with a cup size B. (AA is a 0-inch difference, A is one-inch, etc etc). Using this math, your main bra size is a 38B.

A sister bra size chart of bralette sizes for WAMA underwear.

So, now that you know your standard bra size, how do you find your possible sister sizes? You can use a sister bra size chart, or a size chart provided by the brand (see the WAMA size chart for our hemp bra, pictured above!). On this chart, for example, if your standard bra size is 40DD, one of your sister bra sizes would be 42D, which means you might be able to wear a 2X-Large OR a 3X-Large. Options! Yay!

You can also use more math (yay?). It’s actually fairly intuitive: if you’re increasing the band size, you’re decreasing the cup size, and vice versa. You also don’t have to limit yourself to one above or one below—which means even more options! Using the chart above again, if you’re a 38B, you’re also a 40A, or a 36C, or a 34D, or even a 32DD (which isn’t an option on the chart, but you get the point).


A woman looks away into a bathroom mirror, her hair damp, as she wears a black hemp bralette.

As with all bras, keep an eye on a few things to make sure the bra is fitting you properly. The eternal question, how should a bra fit, can be answered by checking the list below:

  • Cups: No overflow, gaping or wrinkling. Your breasts should fill those cups but not overfill. Also, wrinkling of the cups tends to mean that there’s too much fabric, meaning they’re too big.

  • Band: No riding up or squeezing, and the center of your bra should be flush with your chest. You shouldn’t be able to see your torso or feet through your bra, but you also shouldn’t have any uncomfy bulges. Also, if the band is riding up at the back, it’s probably too loose.

  • Straps: No slipping down or digging in—they should fit well and not need constant adjustment. Don’t forget that the straps can be adjusted, though! I always forget this and it can be a quick fix.

  • Comfort: if the bra hurts you in any way, it’s gotta go! Although sometimes bras do need a day or two of wear to fit super well, they shouldn’t cause any pain from the get-go. I’m talking in the back, neck, boobs, anywhere. And if anything is jabbing or poking, that also won’t improve with a few days' wear, so ditch that size.


A close-up of a Black woman closing the hook of a black, strappy bra on her back.
  • Check the hook of your OG bra. If your current bra is hooked on the furthest clasp, try the sister bra size up. If it’s hooked on the shortest clasp, try the sister bra size down.

  • If the band feels fine, check the cups. If the cups are overflowing, try the sister size up. If the cups are gaping, go down.

  • Check the band and cup size. If they both feel too small, try the sister sizes up. If they both feel too big, try the sister sizes down.

  • Shop bralettes. Since bralettes come in fewer sizes, generally small, medium, large and so on, it can be easier to measure yourself, and find that elusive bra comfort.

  • Try going up or down just one sister size first. Although the sister sizes will all technically fit you, sizes that are the closest to what you measure will likely fit best.

Now that you know your bra sister sizes, I hope bra shopping is just a tiny bit easier for ya. Anything to make that experience better is a win in my book!

Have you tried out your bra sister size(s)? Have they worked for you? Sound off in the comments!

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