Finally, Bra Sizes Explained

Model wearing black t-shirt bra

How are Bra Sizes Determined?

On top of being highly technical garments, bras are also complicated in the way their sizing works. For example, a woman who wears a 32G cup isn’t necessarily a true G cup. Confusing, right? But stay with us — we’ll explain how bra sizes work in just a bit. It’s no wonder that 80 percent of women are wearing the wrong size when one’s cup letter doesn’t directly translate to their cup volume. See why your measured bra size may not be your actual bra size.

How to Find Your Bra Size

First things first, let’s talk about sizing a bra the traditional way. Bra size measurements are calculated based on your band size (number) and cup size (letter), which are typically determined by using a tape measure around your rib cage and bust. If you’d rather skip the dressing room, ThirdLove offers an online Fitting Room Quiz that matches you with your bra size and recommended style based on your unique breast shape. Regardless of what method you choose, you’re left with a number and letter that is your bra size.

How to Determine Bra Cup Size

Now, here’s where things can get a little tricky. For reference, any cup with a 34 band size is considered a “true cup” size. This means that a 34AA is in fact a AA cup, just as how a 34B is a true B cup. However, a 34B is also equivalent in cup volume to a 30D, 32C, and a 36A. All three aforementioned sizes are a B cup despite what their cup letter indicates. This is called sister sizes.

Take a look at our bra size chart below for reference. All bra sizes in the same row across share the same sister size, aka they’re all equal in cup volume. Let’s do another bra cup size comparison: if you wear a 32D, you are actually a true C cup. An easy way to determine your true cup size is simply to locate your bra size on the chart below and see which cup letter is paired with a 34 band in your row.

A chart showing bra sizes and their sister sizes.

The Importance of Sister Sizing

Knowing your sister size can be helpful when adjusting your bra size. For example, if the cups fit comfortably, but the current band size is too loose, you’ll have to try a smaller band size as well as a larger cup size to maintain the same cup volume. So if you were in a 36F, going down in the band size alone would result in a cup size too small. Though the 34F sounds like it has the same cup volume as the 36F, it’s actually one cup size smaller. To keep the cup volume the same, you’ll need to try a 34G, which ensures a snug and comfortable fit, while maintaining the same cup volume as the 36F.

If you’re still not following along (you’re not alone!), you can always take our quick online Fitting Room Quiz to figure out your perfect fit.