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Two women High-five high ten in sports bras looking happy

Understanding Sister Sizing to Find the Perfect Sports Bra Fit

Finding your perfect sports bra size can be tricky. However, it's vital for providing support and protection for the breasts. While finding the perfect sports bra can be a challenge, many women have become savvy to "sister sizing" as a way to easily and quickly determine how to change their size correctly to get the best fit.

What is 'Sister Sizing'?

Sister sizing refers to the idea that bras of different sizes may fit similarly, even though the numbers and letters on the label are different. This is because bra sizes are not uniform across different brands and styles. Additionally, they can vary based on factors like the type of fabric used and the design of the straps and band.

Sister Sizing Charts & Calculators

A sister sizing bra chart is a visual representation of how different band and cup sizes relate to each other. It typically includes a grid with band sizes running horizontally and cup sizes running vertically. The intersections of the grid indicate the different sister sizes. For example, a 34B and a 36A are considered sister sizes because they have the same cup volume (34B = 36A) but different band sizes. This means that a woman who typically wears a 34B may be able to comfortably wear a 36A sports bra, and vice versa, if the band size needs to change. This chart can be a useful tool for determining what sister sizes may fit you based on your usual size.

The vertical coloured columns of this chart represent the sizes which have the same cup volume.

MAAREE Bra Grading Siszter Sizing Chart

On some websites you can also find size calculators, which are online tools that can give you an idea of your size based on your measurements. You simply enter your band measurement and cup measurement, and the calculator will provide your calculated size. Combined with the chart above, this can be an easy and convenient way to quickly determine your size or potential sister sizes.

If you go up in one, go down in the other

The key to sister sizing is understanding the relationship between the band size and the cup size. As the band size gets larger, the cup size gets smaller. So, a woman who wears a 36B may be able to comfortably wear a 34C, as the band size gets larger (36 to 34) the cup size gets smaller (B to C).

The Benefits of Knowing Your Sister Size

Sister sizing can be especially helpful for women who have difficulty finding a bra or sports bra from a new brand that fits well in their usual size. For example, if a woman typically wears a 34B but finds that the band is too loose or too big but the cup size is good, she may want to try a sister size like a 36A or a 32C.

See this visual explanation taken from the MAAREE Instagram Page. Follow us on socials to learn more helpful tips.

MAAREE Bra Grading Explanation

Understanding how sister sizing works can help many people to realise how bra sizing actually works, which is commonly misunderstood: the letter on the end of size doesn't actually represent a specific volume of breast. A 'DD' will be a completely different volume on a 32 size band compared to a 'DD' on a 38 size band.

It's important to note, however, that sister sizing is not a one-size-fits-all solution. Every woman's body is unique, and what works for one person may not work for another. It's also worth noting that while sister sizing can be a helpful tool, it's not a replacement for a proper fitting, especially if you struggle to identify the signs of an ill-fitting bra.


Sports bras are an essential piece of athletic wear for women and sister sizing is a helpful tool in finding the perfect fit. By fully understanding how bra sizing and sister sizing works can empower us to shop confidently for our bras. To get the best fit possible, it's always a good idea to get measured by a professional bra fitter from the brand you are purchasing your new bra with. They can help you determine your correct size and recommend styles that will work for your body type.

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