Did you know that the difference between a properly and poorly fitting bra often comes down to minor nuances? Bra sizing seems pretty straightforward — the band measures inches around the chest while the letter-graded cups increase according to the volume of breasts. Easy, right? Well, turns out there are a lot more intricacies involved which have been put in place to create more choices for a broader sizing spectrum. After all, women are endowed with a variety of breast shapes, densities and volumes, not to mention body types and support needs. The more you know up front, the likelier you are to choose the size that’s best for you.

According to a fit study, about 80% of women surveyed were wearing the wrong bra size. That figure is a high one because most of us don’t realize that band width directly impacts the cup’s volume. So, for example, a 36D is actually equivalent in cup size to a 34F, because an increase in inches also dials up the capacity of the cup. If a woman needs to downsize her band, her cup will require a letter upgrade, too, so that the resulting fit will be more suited to her body.

How Cup Sizes Work

Standard to all bra sizing is something called a “true cup,” which is any bra with a 34-inch band. This means that a 34A is a true A, a 34B is a true B, and so on up through the letter sizes. But, because band and cup sizes parallel each other, a relationship known as “sister sizing,” the capacity of a 34B bra is also equivalent to that of a 30B, a 32C and a 36A.

Why is this important? Say you try on a bra with cups that appear to fit well, but the band’s a bit loose. So, the next bra you try is tighter for what you presume will be a better fit, but… now the cups are too small! This is when knowing your cup’s sister size will help, because you’ll be able to accurately determine the right cup letter as it corresponds to your band’s width — which may fluctuate up or down depending on what feels most comfortable on your body.

Cup Size Chart

Wondering whether you’re wearing a true cup size? Check out the chart below. All of the sizes in a single row share the same sister size. All you have to do is locate your bra size on the chart, then scan that row to see which cup letter falls under the 34-column, listed at the top.

How Band Sizes Work

Bra support mainly comes from the underband, so naturally, wearing the wrong band size almost always translates into an uncomfortable fit. The measurement is based on the inches around your rib cage, just below the breasts. Keep in mind, this is different from how the bustline is calculated — that number comes from measuring around the fullest part of the breasts.

Band Size Chart

Wondering which “true band” size matches up to your cup size? Find your bra size on the chart below, then scan the row to see which cup letter falls under the 34-column, listed at the top.

Sister Sizes

What are sister sizes?

Simply put, there are small but sometimes noticeable differences in fit due to changes in band size. In order to correctly determine bra size, the general rule is that when the band number goes up, then the cup letter may need to come down, and vice versa. As discussed, this is called sister sizing and it helps ensure the best possible fit by broadening the bra-size playing field, so to speak. Because bra sizes also differ from style to style, and even manufacturer to manufacturer — not to mention that women have unique bodies and proportions, it makes sense to try on several bras in your sister-size range.

How to calculate your sister size

The key to sizing up or down a cup is to also adjust for the width of the underband accordingly. Here’s an example for reference: a size 34D and 36D bra do not have the same cup capacity, but a 34D and a 36C do.

Bra Sizes US vs UK

Overall, bra sizes in the US and UK are similar within the AA-DD range. For larger sizes, the US uses this format: DDD/F, DDDD/G, H, I, J, K, L, M, N, and O. However, in the UK, the letter grades begin to differ — after the DD mark, comes E, F, FF, G, GG, H, HH, J, JJ, and K. The highest American cup size is typically an O and that equates to a K in the United Kingdom — that’s a pretty big letter difference, so it’s important to get your conversions accurate up front before buying bras from across the pond.

Giving expert bra advice is kind of our thing, so don’t hesitate to reach out if you still have questions. Unsure about your own band and cup size combination? Check out How To Measure Bra Size to narrow down your best-fitting bra size and great styles recommended just for you. Happy shopping!