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To go up a band and down a cup, or vice versa

You use sister sizing when the cups of the bra fit but the band is a little off. You could also use it when your exact size isn't available, so going up a band and down a cup might work (to sister up in bra size).

Sister sizing refers to the set bra sizes that are believed to hold a equivalent cup fit or similar volume in different bands.

To find any bra's sister size you need to increase the cup size (the letter) each time the band size (the number) goes down or vice versa.

  1. Examples:
  2. When to sister size
    1. Limitations
    2. Sister up
    3. Sister down
  3. Visual chart
  4. Using index sizes
  5. On breast volume
  6. Frequently asked questions
    1. What if the bra uses US, EU or a different sizing system?
    2. How do you know how the cups go?
    3. Can you sister size between brands or models?
    4. How many times can you sister size? Two, three?


Sizes 36C and 34D would be sister sizes, because they will maintain a similar cup fit, but changing in bands. If you needed to buy a 34D and there isn't any available, you could probably try the 36C since it is its sister size up instead of trying the 36D which would probably be too big in the cups.

Here is a list of how sister sizes would extend across many sizes. This means that in theory these bras would have a very similar underwire piece for them.

When to sister size

Sister sizing is useful when a bra's fit is not quite good, but sister sizing up or down could help.

You may need to do this if you are trying to fit into a bra that does not come in your precise size but does in one cup smaller, on this case you will need to sister size up, but keep in mind this isn't your ideal size so be ready for some small compromises.


It is not recommended to sister size more than two steps on any direction.

Ideally you'd only do it once and try a 2nd time but beyond two jumps the general proportions of the bra have changed much and the rest of the bra will not be a good fit.

Be wary about applying sister sizing across brands or models. Grading will vary and sister sizing will most likely won't work as expected.

Sister sizing may introduce other fit issues as you switch sizes, mostly related to the frontal part of the cup such as gaping or quadding. Some brands will have cups that truly are the same in sister sizes, while some brands may make more changes to the exact size and shape of the cups as the band sizes change.

Sister up

Sister sizing up means to increase the band size, thus decreasing the cup accordingly. Do this when a bra almost fits you but the band is too tight. Sister sizing up will give you a bigger band while maintaining most of the cup fit. It may have a slightly shallower cup but not by much so it's worth the try.

For example, starting with a size of 34:5, sister sizing up one step would result in 36:4, doing it again in 38:3.

Sister down

Sister sizing down means to go down on band sizes and up in cups. Do this when a bra almost fits but the band is on the loose end but the wires and cup's fit is good. Sister sizing down will give you a smaller band while maintaining the main cup's construction and very similar fit. If anything it may have a slightly deeper cup.

For example, starting with a size of 34:5, sister sizing down one step would result in 32:6, doing it again in 30:7.

Visual chart

Using index sizes

Using index sizes, you can count how many cups on each direction you want to go and then just add or subtract from your current cup index.

Starting on a 28:10 cup, if you were to sister up 3 bands to a 34 band, you know you need to subtract 3 from the cup index 10, which would result on 34:7.

On breast volume

It is commonly said that sister sizes hold the same breast volume, and it is an easy way to understand it. But what really happens is that sister sizing means that the same underwire piece has been used for those bras, thus the cup fit will be similar but the overall bust perimeter used as reference will have changed a bit making cups shallower or deeper.

Frequently asked questions

What if the bra uses US, EU or a different sizing system?

It does not matter which sizing system the bra you are dealing with is using. What matters is that you always keep withing the same sizing.

How do you know how the cups go?

Good question, the problem is that it is possible that you don't know if for a given brand the next cup after DD will be DDD or E. Here on Bratabase we keep track of the sizing of each brand so you can easily know how do the cups go.

We recommend trying to use index sizes ;).

Can you sister size between brands or models?

Well, in theory you can. Nothing stops you from mathing your way away. But the fit and grading on each brand will vary making sister sizing across models and specially brands very inaccurate. It is recommended to use it when staying within the same bra model.

How many times can you sister size? Two, three?

We recommend not doing it more than two times unless you really need to (You are really out of the bra's available size range).

Sister sizing more than two times may require altering the bra. Probably using an bra extender or shortening the bra's band.

Chances are that you are increasing your band in order to get a smaller cup, doing it much you'd be going backwards into the most common bra sizing problem: wearing a too big band.