Everything You Need To Know About Bra Sister Sizes

There’s a lot of confusion when it comes to bra sizing. 

I know that when I was younger,  I definitely had a hard time with this concept. Like most of you, I went to your everyday department store and picked out the bra I figured fit me the best.

However, I was constantly adjusting it. I would be pulling down the back of my bra while lifting up the front. To compensate, I tried tightening the straps super tight, which only made the bra even more uncomfortable. There wasn’t any support. 

But I felt like I had no other options. I wasn’t taught this stuff.

Can anyone else relate?

The department stores only carry bras that they think are typical according to US standards. There is a very limited selection. This is one reason why so many women wear the wrong bra size. For many years, I was a 36DD because that is what they had available in stores.

This is nowhere close to my actual size.

Sister sizing is a great place to start learning what size you actually are. Here are some tips for finding out your correct bra size using sister sizing.

What Are Bra Sister Sizes?

There’s a huge misconception that all cup sizes are created equal when in reality that’s not the case.

Sister sizes are alternate bra sizes where the cup volume remains the same regardless of the band size and cup letter.

For example, a 30F and a 38C are technically the same size because the cup volume is the same. 

Someone who is a 38DD has more breast volume than someone that is a 30DD. In other words, the 38DD has bigger boobs even though the cup size is the same letter.

30DD and 38DD are not equivalent sizes.

Of course, this is confusing for mostly everyone!

How Do Sister Sizes Work?

The band size affects the cup size. If you need a band size that is smaller, then the cup size will increase. Vice versa, if you need a bigger band, then the cup size will decrease.

Pretend you wear a 34DD.

If your band is loose, you will need reduce the band size, making your new band size 32. If your breasts fit in the cups beforehand (sometimes it can be difficult to tell when your band is really loose), your new cup size would be an E. Ergo, your new bra size is a 32E.

I know you are probably thinking…if I’m a DD and my band is too loose, why wouldn’t I just wear a 32DD?

Because the cups are dependant on the size of the band, the sister size of a 32DD is a 34D, not a 34DD. Therefore the cups would be too small if you were to wear a 32DD.

If you need to reduce down two band sizes then you would increase two cup sizes to become 30F and so on.

If you need to increase your band size, then you will need to decrease your cup size. One size up would be a 36D and two sizes up would be a 38C.

If you are wearing the wrong size completely, please refer to my article 5 Signs You Are Wearing the Wrong Bra Size. In my case, I was wearing a band that was two sizes too big and needed to go up three cup sizes.