What is Sister Sizing?

The concept of 'sister sizing' seems simple, but is something many people know little about. Knowing how to work out alternative bra sizes you can wear can be really helpful when trying on bras, allowing you to find the perfect fit, and solving any issues you may face if your true size isn't available. 

What is Sister Sizing?

Sister sizes are alternative bra sizes to your current bra size where the cup volume stays the same even though the band size and cup letter change. Everyone has more than one size option when it comes to buying bras, these are called Sister Sizes, you can sister size up or down from your usual size to ensure you are getting a bra that fits comfortably. This may also give you the freedom to shop with brands you thought previously didn't cater to your size or whose version of your true size didn't fit well.

How does sister sizing work?

Before discussing sister sizing, it is helpful to understand how bras are usually sized. This differs from country to country, but as a rule, cups sizes are represented by a letter (such as A, B or C) and band sizes by a number (32, 34 or 36). 

The band size is a direct translation of your under-bust measurement and the cup is worked out in relation to that. The cup size is a simplified way of labelling the volume of your bust in relation to your under-bust (the area where your bra under-band sits).

Band sizes and cup sizes are relative to one another, meaning a C cup, for example, is not the same across band sizes. This is because the cup volume adjusts with the band size. For example, someone who is a 36C will have a noticeably fuller bust than someone who is a 30C, the capacity of a cup increases with the band measurement so you cannot wear the same cup size regardless of band measurement when looking to find an alternative size. 

Fleur of England Kyra Balcony bra close up

Do I need to know my cup size to find my sister size?

Yes, Sister sizes are sizes that have the same cup capacity but a different band size. For example, someone who wears a 32C could also wear a 30D and a 34B depending on if they want a tighter or looser fitting bra. Make sure you have been fitted and know your true cup size before trying to work out sister sizes. If you want to understand why your bra might not be fitting perfectly, check out our blog here.


Example: If you’ve been fitted and told that you’re a 34C and find that the fit is good but feel you would not lose support if the underband were a little looser, you could try a 36B. This gives you a more relaxed fit in the underband, but with the same cup volume as the 34C. If you believe you could afford to have your under-band be a little tighter then you could also try a 32D. It is useful to remember that, while your under-band should not feel restrictive or tight, it does need to be firm for the correct support so be careful when opting for a looser under-band if you plan on everyday wear.


 Fleur of England Gisele Strap Balcony bra shown on model

If my size is sold out can I buy a different size?

Have you ever found luxury lingerie that you adore but your size is out of stock? Don't worry you might be able to shop another size. If your size isn't available you can try a size within your sister size range, sister sizes are a great tool if your normal size is sold out or a certain style isn't fitting right. If neither are fitting well it might be that the style isn't quite right for your shape.

Sister Sizing Chart:

Each pink line represents the same cup volume and Sister Size.

A simple way to remember is when you go up in the band, you go down in the cup, and when you go down in the band you go up in the cup.

Should I Sister Size up or down?

  • Sister Size Down - If your bra is on the tightest hook but still feels comfortably loose, you can sister size down. This is one band size smaller than your current band size and one cup size larger than your current Cup. For example, if you wear a 34C, your Sister Size down would be 32D.

  • Sister Size Up - If your bra is on the loosest hook but feels firm, you can sister size up. This is one band size larger than your current band size and one cup size smaller than your current Cup. For example, if you wear a 34C, your Sister Size UP would be 36B.

Before you use a bra as the point of reference for sister sizing, it's worth considering the age of that bra. Over time bras will loosen and lose their shape with wear so, while the underband on an older bra may feel loose and comfortable, you may find that sizing down in the under band on a new bra can feel like a big change. You may also find that, while sizing up may seem like a bad idea, with a brand new bra it will be firmer than you think. Check out our guide on whether it's time to buy a new bra here.

Remember, all brands and styles of bra will fit slightly differently, so it's a good idea to try a few different sizes to find what works best for you. The most important thing is that you feel exquisitely comfortable in lingerie which makes you feel empowered.



Want more tips for shopping?

Check out our blog here for tips on buying briefs. You can also find our blog on how to shop more sustainably here.