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When talking about bra sizes, we will be referring about bras that use letter and numbers for sizing. Usually start with a number followed a letter(that can be repeated), a few examples:

The number indicates the band size of a bra. This makes reference to your the size of your torso/ribcage right below your breasts. The numbers used can vary between regions or brands. The above examples use US/UK sizing (based on inches).

The letters are used to determine the cup size of the bra. This is relates to the breast mass, colloquially, "how big" your boobs are. The farther along in the alphabet, the bigger the cup. The way that the cup letters go up will vary between brands, some brands will use cups DDD, others will use E cups and so on (this makes things complicated on bigger sizes).

The letter to use indicates how much bigger the full bust measurement is with respect to the band size. This means that the band size and the cup size are dependent of each other so a cup size is always needs a band size to indicate how big or small breasts are. There is no "D" cup, since it will fit very different breasts in a 28 band than in a 40 band.

  1. Band size
    1. Adding inches to your torso to calculate your band size
  2. Cup size
    1. Different grading
  3. Sample bra sizes table
  4. Sister sizes
  5. Different sizing systems
  6. Size range available
  7. Calculators
  8. Frequently asked questions
    1. Is it true that double letters mean half cups?
    2. If I wear a ##XX size on brand Y what size I am on brand Z?
  9. Resources and further reading

Band size

The band size is determined by a number which is related to the circumference of your torso.

Most bands will measure, in inches, a bit less than the size they are, but stretch past that number. For example, a 32 band might measure 28 inches unstretched, and stretch to 35 inches. Different brands and styles may have more or less stretchy bands.

Most women find that the best fit is the band size closest to their underbust measurement (in inches). The best fitting band size will depend on how squishy your tissue is where the band sits, and the weight of your breasts. There is some amount of personal preference involved in this too, as some women find they like a firmer band and some like the band to be a bit looser.

Adding inches to your torso to calculate your band size

Early bras were produced before elastic material and, for some reason, the old measurement tables are still being used in some places, even though nowadays bra bands are made of elastic material and therefore stretch.

This easily explains why the +4/+5-sizing method doesn’t work very well if you want your bra band to support you: the band stretches out too much! You can test that by stretching out a band from a +4/+5-fitted bra - they will often stretch absurdly far.

Cup size

The cup size is a letter which is determined by the difference between the full-bust and the band measurement. For most brands, each inch difference will increment one cup size.

A cup size always depends on the band length! You can't only say "I wear a D-cup" because a 34D and a 42D are totally different sizes!

A person who wears a bra in the size 34D has a smaller breast circumference than a person who wears a bra in the size 42D. Because cup size is relative to the band size, all this means is that they have the same difference between their breast circumference and the band length.

See these resources to read more about this:

Different grading

Not all brands will use one inch increments to determine their cups. Some brands will use 2cm while others could use 4 cm between cups (Avocado).

This makes it very difficult to know what size you will be between brands, this is why on Bratabase we recommend always going by the bra's measurements since labeled sizes cannot always be used.

Sample bra sizes table

Sister sizes

Two sizes are considered sister sizes when their bands are consecutive but their cups maintain a similar fit. Usually, a sister size is found when adding or subtracting one cup when going up or down one band size.

This can be explained when you consider that a cup size is always determined by a difference between band and full-bust measurement.

Learn more about sister sizes

Different sizing systems

There are a lot of different bra sizing systems, some of which can be converted to other systems and some of which cannot. Sometimes the sizing systems are applied consistently or they might vary between brands or even models of one brand!

More details about different sizing systems

Size range available

The actual size range available might be much bigger than you imagine. On Bratabase, bras in sizes from 26D to 50N have been added and this size spectrum is ever expanding!

On bra models which have been added to Bratabase there's a field which shows you in which sizes this bra is being made.


There are a number of different bra size calculators on the web. Try for example the bra calculator on Sophisticated Pair. It can give you an idea of where to start when you want to try out a new size. (You should probably choose the loose band setting if you're new to bra-fitting. You can always try on shorter bands if they don't feel snug enough.) Please don't give up immediately if the recommended sizes sound really odd and way too big - don't be a victim of letterphobia. :)

Keep in mind though: A calculator can always only give you an idea. It never says the whole truth, a lot of us need different sizes in different brands or on different parts of our menstrual cycle.

Check the article on right fit of bras to find out how to tell if your bra fits, and how to find a better size if it doesn't.

Frequently asked questions

Is it true that double letters mean half cups?

Short answer: False, double letters are just one more cup increment. The cup increment between an B and C cup is normally the same as from FF and G cup (one inch different between full bust and under bust measurements).

Long answer: Remember that each manufacturer has freedom to do whatever they want to their sizes, so it is entirely possible that some brand will claim that they do half cups (Thirdlove & co, Lady Cameo).

If I wear a ##XX size on brand Y what size I am on brand Z?

You have to try it. If you wear a 38F on one brand, this should be the first size you try on another brand if they have the same size available and see from there.

If the brands use different sizing systems, or even worse, different grading systems. Then it will be inaccurate to make just a straight conversion to find your fitting bra. We recommend doing a comparison by bra measurements to get an idea. For example if you wear a 38H in Freya, chances are that a 85H will fit not fit you in Comexim because these two brands not only use different band and cup sizing, but also use different cup grading, so any conversion will be off.

But, what if I count the cups, could I make the conversion?

This is what Index sizes is. Read its limitations to know why it doesn't always work :)

Resources and further reading