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Sister Size Sports Bras Three identical bras of different size

What is 'Sister Size' and how is it different from your bra size?

Heard of 'Sister Size' before but don't know how it relates to your bra size? Not sure how a bra fitting works? Let us shed some light on the topic.

If I tell you to imagine a cup size D, what do you picture? Do you think of a specific size or volume of breast? I have spoken to women who didn't realise that if you pick up a bra in a size 32D and one in a size 38D you will get cups of different volumes.

So, what bra size am I?

Bra fitting MAAREE

First we measure the underband.

Start by measuring the bra's underband size. To do this you will need to place the tape measure flat, snug and level just beneath where your bra sits and read the measurement in inches. Next, if the measurement you just read is an even number then this is your underband size. If it is an odd number you must add one inch to this. This new number is your underband size.

Next we will measure the cup size.

Bra Bust and Underband Measurement Locations[/caption] Use the same technique again, but this time around the bust. This is the fullest part of your breasts. Now time to compare this bust measurement to your underband measurement. Your cup size is determined simply by comparing the difference in measurements (in inches) between your underband and cup sizes. Who would have known.... A cup is a difference of +1 inch. B cup is a difference of +2 inches. C cup is a difference of +3 inches. D cup is a difference of +4 inches. And so on... For example, if you measure your underband size to be 32 inches and your bust size to be 36 inches, your bra size will be 32D.

In that case, what sports bra size am I?

When it comes to sports bras, size is determined by the measurements above. However, personal preference plays a part also. Some women like to have their sports bra fitting tighter, whilst others prefer more breathing room. It is a misconception, that when you desire a tighter or looser fit to keep the cup size the same and change the underband size. This is where you will need to know your sister sizes. So read on.
One important thing to keep in mind, sports bra shopping is the same as clothes shopping. You are not necessarily the same size in ever shop. Some bras will fit and feel differently as some will have more elastic than others and all will be constructed differently. So make sure you read how that brand recommend you should measure for their size chart. Most importantly, you should know the signs of a good (and bad) fitting bra. This will allow you to recognise if it's the correct fit for you.

Sister Sizes Explained

Not sure what sister size is

If you're like me and experience that you can find bras of the same size from different shops fitting differently. You might and want to go up or down a size, this is how to do so. Take your size from the measurements taken above. If you desire a looser fit on the underband, increase your underband size by 2" but reduce your cup size by one letter. For example, if you are a 34D in this scenario, then your sister size is 36C.

On the other hand, if you find that the band it too loose then you should take the opposite steps, reduce your underband by 2" and increase your cup size one letter and pick up your sister size of 32DD. If you were to pick a different size underband but keep the cup size to the same letter then you will notice this volume get larger and smaller too. This technique will alter your underband size but keep your cup volume consistent.

Do you feel like there's enough information out there about bra sizes? Do you have any further confusion around sports bras? Let us know in the comments below or on Instagram.

For further info or guides to sizing for MAAREE clothing, visit our Size Guidelines page.

Do you know what you're looking for when shopping for a sports bra? Do you know the difference between an Encapsulation sports bra and a Compression sports bra? Learn more.



Can’t seem to find anything for bigger women as myself, cup sizes are there but not bigger back sizing, (ie past a 38) is there more to come?

Martina Rafferty

I am looking for a sports bra that fits my 36DD breast but sits flat on the side I have had a mastectomy. I have not had reconstruction and I don’t wear a prosthetic. Would your sports bra work for me and if so, how should I work out the sizing please?


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