What Are Bra Sister Sizes and Why Do They Work?

You may have heard lingerie fans criticizing the idea of “sister sizes” in bras as a general bad practice. That wearing a sister size is “wrong,” won't be a good fit, and is just a way for brands to sell you more product.

However, sister sizing isn’t a negative concept. At its core, it’s a neutral fact describing how bra sizes work.

So what is a sister size? How do you find yours? And when is it actually super beneficial to your lingerie shopping experience?

How does sister sizing work?

A “sister size” is a size that has the same cup volume, but a different band length, as another size.

So if a bra is a size 32B, its sister size “up” would be 34A, and its sister size “down” would be 30C. Despite having different numbers, these bras all hold the same amount of tissue in their cups, with different band lengths.

Why do sister sizes have a bad reputation?

Most people’s first experience with sister sizing is in a bra boutique that doesn’t carry or stock their size. They might hear, “We’re out of the 32C, but you can wear the 34B!” Or, “You measure at a 28D, but no one makes that size, so you are a 32B!”

In this case, the sister size is seen as being used to make a sale, rather than to benefit the bra-wearer. Boutiques and brands might encourage sister sizing to ensure the customer leaves with something, or to build loyalty to their company, instead of sending the customer somewhere they will actually find their size.

And say you're someone who wears a fuller cup size, but is sister sizing into a bra that isn't designed for a fuller cup size. For instance, you normally wear a 30FF in full bust brands but want to try a 36DD in a core-size fashion brand. In this case, the cups might technically fit, but you won't get the same amount of support from the band (because it's too big) or the cup (because it's designed for someone with less extreme proportions).

This makes sister sizing automatically feel like the wrong way to go. However, sister sizing isn’t always a bad thing.

When is sister sizing helpful?

• When your bra band doesn’t fit. If the bra is riding up, you can sister size down to keep your cup volume the same but add more support through the band. If the bra is so small you can’t even fasten it but the cups are great, you can sister size up to feel more comfortable. In both cases, the cup volume will be the same.
• When a bra is known to run small or large. Just like in clothing, bras do not have a standardized size system. Different brands use different measuring systems and size charts. Plus, there are often variations between styles for the same brand. If you’re shopping online and know a certain bra runs small in the band, you can order a sister size up to get a better fit.
• When you love a bra but it doesn’t come in your preferred size. I like to wear a 32G, but I sister size to a 36E or DD  when I want to wear brands like Agent Provocateur and Chantal Thomass. I won’t get the same amount of support as I do in my preferred size, but I get to wear some really cute things that I otherwise would miss out on!!
• When you’re buying a gift. If you’re absolutely set on a certain bra for your recipient but it isn’t made in their size, picking a sister size is a (slightly risky) way to give the gift you really want to give—especially if the bra is a novelty style that isn’t meant to be supportive.

There are so many shadows around the world of bra fit, but you don’t have to be a professional fit expert to get the basics of how bra sizing works. Understanding sister sizing helps you be a smarter consumer, ensuring your next lingerie shopping experience is as easy and fun as possible.