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Bra Knowledge » Sister Sizes » Versions » Sister Sizes as of 2015-09-16 10:40:57.398321

Sister sizing refers to the set bra sizes that are believed to hold a similar breast volume.

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

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.

Sister up

Sister sizing up means to increase the band size, thus decreasing the cup accordingly.

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.

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.


It is not recommended to do sister sizing 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.

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?