Sister Size Calculator


The power of sisters: how your bra sister size can transform your lingerie experience


Chances are, you learned somewhere along the line that you had a bra size and that was the end of the story. You may have also thought that unless your breasts actually changed size due to weight, age or general lifestyle, there was no way someone who was, for example, a C cup could also be a D cup.


Well, we’re here to let you in on a bra fitters little secret – bra sizing isn’t fixed like clothing size is, and you can go up and down a cup size and fit perfectly well, as long as you also adjust the band size. It’s called sister sizing, and isn’t as complex as it sounds! Let us explain…


What is sister sizing?

Sister Sizing is the tool that bra fitters use to fit you in different bras with different bra band sizes. In changing this band size, the fitter will also affect the cup size, i.e. how much breast tissue the cup can actually hold. This happens because the band and cup sizes are ratios of one another – when one increases, the other decreases.


If you take a look at the chart below, each of the sizes has been put alongside its sister size group in horizontal rows and each size will hold the same volume of breast tissue as every other size in that row. That’s not to say that someone who normally wears a 26D would look or feel good in a 32A, because chances are that their breasts won’t be properly supported! This is where a little common sense comes in, because sister sizing is best kept to one size either side of your normal bra size. This is important because sister sizes are key to finding a well-fitting bra.


 Sister size chart


How do I find my sister size?

For example, let’s say you are trying on a bra in a 36E and wanted to adjust the band slightly for a better fit. For a slightly tighter band, you’d move down to a 34 inch band. If you wanted something a touch looser, you’d size up to a 38 inch band. So far, so simple!


But what should you do with the cup size? Well, because you have adjusted your band size, you also need to adjust your cup size in the opposite direction by the same number of sizes. For example, going from a 36E to a tighter band would mean you need a 34F, and upsizing for a looser band would mean you’d be wearing a 38DD.



Example: your cup fits but you need to change the band size

In the example below, the band size changes to ensure a good fit around your chest. Because the cup size fits and holds your breasts securely, you need to also amend the cup size in order for it to hold the same volume of breast tissue. As the band size increases, the cup size decreases.



Translating that to our size chart - to keep cup volume the same, read the size chart horizontally. Remember not to go overboard with sister sizing, otherwise you won't feel supported or have forward projection and uplift. Always keep to one size either side of your actual bra size. 

 Sister size chart




Example: your band fits but you need to change the cup size

In the example below, the cup size changes to ensure a good fit around your breasts. As the cup size increases, it holds more volume of breast tissue even though the band size remains constant.



Translating that to our size chart - to change cup volume but keep the back size constant, read the size chart vertically.

 Sister size chart



Remember though, sisters are similar, not exactly the same. If you find a bra in your sister size that isn’t the best fit for you then skip it. Never compromise on your bra fit – your breasts will thank you for it!




Shop your sister size