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how do bra sizes work

Ever wonder why your bra size changes from brand to brand or style to style? Here’s the reason: Your bra size is more of a guide than a set rule. It gives you an idea of what size you should start with in the fitting room, but it doesn’t mean that you’ll necessarily be taking that size home with you.

We know how complex bra sizing is and how hard it can be to understand. In fact, it involves factors that you might not even be considering when bra shopping, which is why we always recommend getting regular bra fittings and visiting your local lingerie boutique for help. But there’s a chance you’re reading this article because you’re genuinely curious about the world of bra sizing and you’re having trouble making sense of it all. Don’t worry, this article was written for you.

In this guide, we’ll be talking about the basics of bra sizing – how to determine your bra size, how to read your bra size tag, and how to figure out your bra sister size.

Ready to begin? Great! In order to understand how bra sizes work, we must first understand what is needed to determine a bra size.

How to determine a bra size

Your bra size is a combination of a few factors: your underbust measurement, overbust measurement, breast shape, breast root width, and your personal fit preferences, among others. All of these are taken into consideration when determining which bra size will work best for you in any bra you try on. A good bra fitter won’t try to sell you a bra if you feel it’s uncomfortable or too snug. Instead, she’ll consider your personal fit preferences and look for the styles and brands that are comfortable and fit you well.

The underbust and overbust measurements are the two most common measurements taken during a bra fitting. Your underbust measurement is taken below your bust with the tape measure parallel to the floor. Your overbust measurement is taken at the fullest part of the bust, also with the tape measure wrapped around the bust and parallel to the floor. If you’re measuring your bra size at home, these measurements should be taken in front of a mirror so you can make sure everything is lining up properly. For more on what a proper fitting bra looks like, check out our guide here.

How to read a bra size tag

When it comes to bra sizes, the number on the tag is the bra’s band size and the letter on the tag is the bra’s cup size. That being said, the band size is not necessarily your underbust measurement. For example, you might measure 32 inches under your bust, but you may wear a 34 inch bra band. On the other hand, some women prefer a snug band, so they opt for a bra where their underbust measurement and band size is the same number.


Related: Why Do I Wear The Same Bra Size In Every Brand?


What is bra sister sizing?

Bra sister sizing involves finding two alternate bra sizes – one with a band size bigger than yours and the other with a band size smaller than yours. When calculated correctly, you’ll find that your bra sister size has relatively the same cup volume as your current size, but a different band size.

For example, the bra sister sizes for a 34D are 36C and 32DD. The bra sister sizes for a 36DD are 38D and 34DDD/E.

You might buy your bra sister size if you fall in love with a bra that doesn’t come in your regular size or if you don’t like the fit of the band in your regular size.

How to determine your bra sister size

Using our chart here, find your bra size on the bra size chart. Now look for the bra size that is one band size bigger and one cup size smaller than your bra size – this is your bra sister size with a bigger band. Then, starting from the beginning, look for your bra size with one band size smaller and one cup size bigger – this is your bra sister with a smaller band size.

You’ll find that if you wear a 40DD, your bra sister sizes are 38DDD/E and 42D. Anyone who wears a 32C, will find their bra sister sizes in a 34B and 36A.

Don’t have time to convert your bra size to your bra sister size? We’re got you covered. For a quick and easy way to figure out bra sister size (no math required!), check out our helpful bra size chart here.


Related: Q&A: Bra Sister Sizing Frequently Asked Questions


We Highly Recommend

Whether you’re shopping for new everyday bras or lingerie for a special occasion, it’s always a good idea to seek out a second opinion. We know how hard it is to find bras that fit well and feel good, especially when you’re doing it on your own. If you’ve ever felt unsure about your bra size or you just don’t know where to go to find good bras, it’s time to let a bra fitter help.

Many specialty lingerie boutiques offer bra fittings. Their expert bra fitters will take the pain and frustration out of bra shopping and do all the work for you. Even better, their product knowledge can save you time and money. Plus, they know where all of the best bras are hiding.

If you’ve struggled with finding bras in the right size, it’s time to make a change. Visit our specialty store locator to find a store near you and schedule an appointment.

Happy bra shopping!


Featured image lingerie:
Charlotte Padded Bra 6901 | Charlotte High Waist Brief 6917 in Red/Black
Casey Plunge Molded Bra 2801 | Casey Boyshort 2805 in Black/Rose Print
Marion Unlined Wire Bra P5392 | Marion Hipster P5395 in Black

Published by

Katie Donnelly

Katie Donnelly is a full-time freelance writer, content manager, and founder of Katie Rose Creative. An experienced bra fitter, Katie has helped hundreds of people shop for lingerie. Her favorite bra is the Sandrine Plunge Longline Bra.

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