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What are Bra Sizes? What is the alphabet (ABCD) in bra size?

what are bra sizes

Bras are measured using two measurements. The underbust and the overbust. The underbust is the measurement around your ribcage, right below the breast, where your bra band sits. This measurement determines how snugly the bra fits around your body and provides support to your breasts. The band should fit comfortably without digging into your skin or riding up from the back. If it does ride up, you may need to adjust the band and go down a size.

Overbust is around your chest over the highest points of your breasts (The fullest part of your bust that helps calculate the cup size eg: A/B/C/D). The cup essentially describes the volume of your bra cup, this is calculated as the difference between the underbust and the fullest part of your bust(overbust).

The first thing to realize is that a “D cup” doesn’t mean anything without the band size paired with it. For example, a 30D is not only smaller than a 38D in the measurement around the band but also in the capacity of the cup – there is no such thing as a “D” size in itself.


bra sizing

Taking reference from the above information, an example:

In size 34B: 34 is your band size, while B represents your cup size (volume).

The further down the alphabet it is, the higher the volume, which is why AA is the smallest. Some cup sizes have double letters and some single letters, they are based on the different sizing systems followed namely UK/US/EU. They increase equally in alphabetical order, one size at a time, and the doubles aren’t half sizes, it’s just that for some a double letter is used instead of a single. In India, we use UK sizing which tends to have a set method for using the double letters.


DD is one cup size larger than D. After the core cup sizes of A, B, C, and D, the alphabet continues as the sizes get larger, but with some sizes having double letters (such as DD or FF)

There’s no difference in the spacing between single and double letter sizes, a DD is one full size larger than a D, just like an E is one full size larger than a DD.

DDD is most used in the US and converts to an E or F cup in UK sizes – one size up from DD.

Many larger cup bra brands use UK sizing, so it’s worth being familiar with how sizes convert. Our international size charts cover the differences at a glance.

international size chart

What is the difference between cup size & bra size?

cup size and bra size difference

Sister Sizes 

Sister sizes refer to bra sizes with the same cup volume but different band sizes. For example, a 34C and a 36B are sister sizes because both have the same cup volume but with different band sizes. This is helpful to know when trying on bras, as you may find that a sister size provides a more comfortable fit.

Benefits of sister sizes

Time saving

One way is to pinpoint your size in a particular brand much quicker, without needing to try on lots of bras.

If you’re trying on a 38F bra and it’s a bit loose in the band but seems a good fit in the cup, you can use the rules of sister sizes to know that you’ll need to go up a cup letter size when you exchange it for a smaller band, to keep that part the same. So, you’ll switch it for a 36FF, instead of the 36F you might have gone for if you didn’t know about sister sizes – which would have ended up giving you a smaller cup as well as a smaller band.

Size variations

If you find you fluctuate in size a fair amount, whether from hormonal variations affecting breast fullness throughout the month or varying changes in weight over time, sister sizes can help you always have a well-fitting bra on hand. Collecting a “family” of different bra sizes lets you know you’ll always have one you enjoy wearing.

Limitations of sister sizes

Though sister sizes are a great way to open up more understanding around sizing, they do work best while going not more than a size up or down. Beyond that, the garment will have changed enough that it’s not likely it will be able to give you adequate support or a comfortable fit.


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Priyancka Rampal

Priyancka Rampal

Design Manager at Clovia
Priyancka is the Design Manager at Clovia. She is passionate about intimate apparel. In her free time, she loves to delve into the fantasy world of books & attending art-culture events across cities.
Priyancka Rampal

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