If you aren't already familiar with the concept of sister sizing, it's a quite straightforward thing to learn. As we all know bra cup sizes are represented by letters A, B, C, and so on, whereas band sizes are represented with numbers, i.e. 34, 36, 38. We all know what our actual bra size is, but what happens if your standard cup size isn't available or a certain brand doesn't carry your exact band size? Enter sister sizes.
The concept of sister sizes is quite simple to grasp. Keep reading below where we break down everything you need to know about bra sister sizes and how you can find your sister size.
How to Find Your Bra Sister Sizes
When it comes to sizing any bra, it's important to note that the cup volume varies depending on the band size. So, a 38D will not have the same cup volume as a 40D. The band size will always determine how much breast tissue fits in the cup size. To find your bra sister size, all you need to do is go up or down in the band size or cup size, respectively.
Take for example that you wear bra size 38D. If you go up a band size, your sister size with the same cup volume is 40C. If you were to go down in the band size, your sister size would be 36DD. The bra cup volume depends on the band size, so you will never wear the same cup size in a larger or smaller band size than your standard bra size.
Bra Sister Size Chart
| Current Band Size | Current Cup Size | Sister Size (+1 Band, -1 Cup) | Sister Size (-1 Band, +1 Cup) |
|---|---|---|---|
| 30 | A | 32AA | 28B |
| 30 | B | 32A | 28C |
| 30 | C | 32B | 28D |
| 30 | D | 32C | 28DD |
| 32 | AA | 34AAA | 30A |
| 32 | A | 34AA | 30B |
| 32 | B | 34A | 30C |
| 32 | C | 34B | 30D |
| 32 | D | 34C | 30DD |
| 32 | DD | 34D | 30F |
| 34 | AA | 36A | 32AAA |
| 34 | A | 36AA | 32B |
| 34 | B | 36A | 32C |
| 34 | C | 36B | 32D |
| 34 | D | 36C | 32DD |
| 34 | DD | 36D | 32F |
| 34 | F | 36DD | 32G |
| 36 | AA | 38A | 34AAA |
| 36 | A | 38AA | 34B |
| 36 | B | 38A | 34C |
| 36 | C | 38B | 34D |
| 36 | D | 38C | 34DD |
| 36 | DD | 38D | 34F |
| 36 | E | 38DD | 34F |
| 36 | F | 38DD | 34G |
| 38 | A | 40AA | 36B |
| 38 | B | 40A | 36C |
| 38 | C | 40B | 36D |
| 38 | D | 40C | 36DD |
| 38 | DD | 40D | 36F |
| 38 | E | 40DD | 36F |
| 38 | F | 40DD | 36G |
| 40 | A | 42AA | 38B |
| 40 | B | 42A | 38C |
| 40 | C | 42B | 38D |
| 40 | D | 42C | 38DD |
| 40 | DD | 42D | 38F |
| 40 | E | 42DD | 38F |
| 40 | F | 42DD | 38G |
| 42 | A | 44AA | 40B |
| 42 | B | 44A | 40C |
| 42 | C | 44B | 40D |
| 42 | D | 44C | 40DD |
| 42 | DD | 44D | 40F |
| 42 | E | 44DD | 40F |
| 42 | F | 44DD | 40G |
Why Would I Need to Wear a Sister Size Bra?
Knowing your sister sizes is a helpful tool in many different circumstances. Say your favorite lingerie brand released a new style that doesn't come in your usual size. Simply go either down in the band size and up in the cup size, or vice versa, and you'll have a well fitting bra.
Another circumstance where knowing your sister sizes is helpful is if your current bra doesn't fit just right, despite it being the correct size. Here are a few circumstances where you might want to try out a sister size:
If your bra still feels a bit loose on the tightest hook, try out your sister size by going down in the band size and up in the cup size.
If your band size feels a bit snug, try going up in the band size and down in the cup size.
If your breast tissue is overflowing over your bra cups, go up one cup size and down one band size.
Bra sister sizes are helpful in many different circumstances.
When To Use Sister Sizes
Knowing your bra sister sizes is a helpful tool for many different circumstances. Knowing your sister sizing can help you find a better fit, or even wear a bra that you thought wouldn't fit you in the first place.
With that being said, there are always exceptions to sister sizing. Note that sister sizing might not work in every instance, as finding a good fit for you can depend on both the style of bra as well as your particular breast shape.
You should also not use an old, worn out bra to determine whether or not you need to try a sister size. An old bra with a stretched-out band and whose cups fit loosely is not a good point of reference for sister sizing. Bras will inevitably stretch out over time. If a new bra in your usual size doesn't fit just right, that's a good sign to test out your sister sizes.
Final Thoughts
Sister sizes are a helpful tool as this opens up a few different sizes for you to try out when bra shopping. The main goal is to find the best fitting bra for you, so if the best fit happens to be a sister size, that's great! Go with a sister size.
If your current bra size is a perfect fit and you don't have any reason to try out a sister size, that's great, too. The main goal for any bra is having a cup size and band size that fit to your body's unique proportions and provides the maximum support and comfort you need.
If you want to confirm your bra size, we recommend using our Bra Size Calculator. Simply take your band measurement and bust measurement and plug both numbers into the calculator to find your perfect fit.
About the Author
Maeve Gesualdi is a Fordham University alumna where she earned a Bachelor of Arts in Communications and Fashion Studies. Based in New York City, Maeve is part of the marketing team at Glamorise. Maeve regularly interacts with customers via social media and product reviews, as well as with influencers and models. She is constantly getting feedback on our products as well as gaining a better understanding of what women in our category struggle with and/or look for when searching for a bra. Aside from writing, she is passionate about all things travel and fashion.
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