Sister Sizes as of 2015-09-19 00:56:12.116732

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.

You would use this in case you find a bra that fits in the cup but need to correct the band while maintaining cup fit, or if you can't find your exact cup/band size and are willing to try one band up or down while maintaining theoretical cup fit.

Examples:

28HH - 30H - 32GG - 34G - 36FF - 38F - 40E - 42DD (using UK sizing)
40AA - 38A - 36B - 34C - 32D - 30DD - 28DDD (using US sizing)
65H - 70G - 75F - 80E - 85D - 90C (using EU sizing)
38:5 - 36:6 - 34:7 - 32:8 - 30:9 - 28:10 (using index sizing)

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.

Limitations

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.

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.