What is a bra size?
There is NO standard to bra sizing. What brand? What country? Then add to that I see many women wearing bras that do not fit correctly. Or they are old and stretched out. When I am doing breast surgery- whether doing an augmentation or a reduction- being able to accurately discuss breast size is important.
What is the best predictor of breast weight?
For breast reductions, insurance coverage is based on the weight of the specimen removed. It is a weird thing- I can have two patients who I take from a 34DD to a 34C cup. In one patient that change is from removing 150 grams of tissue. In another patient, the change is 500 grams. See all my blogs on breast size.
The breast is made up of fat and breast tissue. Clearly fatty breasts are not as dense and tend to “weigh” less. But is there a better way to predict breast weight?
Published study in Aesthetic Journal
This was a study published in the Aesthetic Surgery Journal, October 2025, “Improving Documentation in Plastic Surgery: Sister Bra Size Group and BMI are More Indicative of Breast Weight Than Cup Size.” This was a study out of UCLA. They wanted to figure out a way to make more accurate documentation of breast size to aid with breast surgery planning, insurance coverage, and research for breast reconstruction. They were frustrated by the inconsistency- many times bra size was not recorded, or if it was, it did not predict breast tissue weight.
What is a sister bra size? This was a new term to me, but it is something that I totally talk about all the time. The “sister” bra size is that a 32 D = 34 C = 36B. That means the same amount of breast tissue is a smaller cup size when the band is bigger. So a 34 C is NOT the same volume of breast as a 36C. It IS the same amount of breast volume as a 32D.
Study:
- Retrospective review of 209 patients (395 breasts) who had mastectomy between 2017 and 2023. (Mastectomy is removal of the entire breast, which then gets weighed).
- Preop bra cup and band, mastectomy specimen weight, BMI, and demographics were recorded.
- They assigned a “sister bra size” to each group. So if you were a sister size 6, that meant you were a 32D/34C/36B/38A/40AA. If you were a sister size 9, you were a 32G/34DDD or F/ 36 DD or E/ 38 D/ 40 C.
Findings?
- Sister bra size group showed the strongest correlation with breast weight, followed by cup size, BMI, and Band size
- Age did not have correlation with mastectomy weight
- When multivariate analysis was done, sister size and BMI were statistically significant predictors of breast weight, where cup and band size were not.
They conclude, “Cup size alone is not a reliable predictor of breast tissue weight.”
Dr. Greenberg’s thoughts
Breast size talk is so important, and so tricky. I do not focus on talking about the ABCDs of bras much in my practice, because there is so much variability between brands. Also I cannot emphasize enough how many women are wearing the wrong sized bra. See my blogs on bras BRA BLOGS
For breast augmentation, when discussing size, I focus on cc’s. I have you try on sizing implants and do the rice test. I do not focus on “what cup size” you will be when done, because of the lack of bra sizing standards.
For breast reductions, at this point in my 25 year career, I am really good at estimating weight of the breast from my exam. It is from years of seeing patients, doing a breast reduction where I weigh the tissue I remove, and then seeing what size they are after. Some breasts are heavier and denser than others. Some breasts compress more (like a minimizer bra) than others. When I got the Vectra 3D imaging I was hoping it would be totally accurate on breast weight. But volume and weight are not the same thing.
So how do my breast reduction patients pick size? There is never a guarantee, and I guide them on likely options, but I have them go buy the bra they want to fit into after surgery. We then review it in the mirror so we both have the same visual of what we are aiming for.
I am interested as this study is trying to make a calculator to estimate how much needs to be removed to hit a target size. I would love to see that calculator. In their chart, a sister size 6 weight went up with BMI. EX: BMI of 24 patient, a sister size of 6 = 34 C = 349 grams. For a BMI patient of 30, a sister size 6 = 36B = 522 grams.
I already am doing breast math in my practice. When I see someone, in my head I estimate “This breast is 900g.” If they want to be around a 36 C, I know that is around 590 grams. So I aim for a 300 gram reduction. It is not a perfect process. I adjust based on the density of what I find inside, and the axillary fat (which is not breast tissue at all but adds to the band size and visual volume of the breast).
I am curious to see what they propose. But I like bringing science to this area. If there were a way to bring breast density into the equation I would love it.
Medical Citation
Improving Documentation in Plastic Surgery: Sister Bra Size Group and BMI Are More Indicative of Breast Weight Than Cup Size, Aesthetic Surgery Journal October 2025
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