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If you’ve ever been lingerie shopping – either in store or online – chances are you will have heard the term ‘sister size’ being thrown around, and we get a lot of customers asking about it. But what exactly is a sister size? Well, I’m going to explain it for you now!

Your sister size is another bra size that will fit you with the same cup volume. This is mostly used for adjusting sizes when the back band feels too small or too tight, but the cup is a perfect fit.

The first thing to remember is that cup sizes are relative to the band size – a DD on a 28 band is much smaller than a DD on a 38 band! In fact, if you were to take a 38DD and keep the same amount of room in the cup on a 28 band, you’d be looking at a 28GG! That means someone who wears a 28GG has the same breast volume as someone who wears 38DD.

The general rule is that if you go up a band size, you need to go down a cup size. The opposite is also true – if you go down a band size, you need to go up a cup size. This ensures the cup size stays the same volume, and only the band size is adjusted.

If you make any adjustments to your band size without also amending your cup size, the volume of the cup will change. For example, if you usually wear a 34FF but find the band is too tight and you try a 36FF, you will end up getting one cup size bigger too!

bra sister sizes cup volume diagram

The cup size or 'volume' will vary depending on the band size it is on.

Measuring in at 28G, my sister sizes would be 30FF > 32F > 34E. With this in mind I know that if a particular brand comes up tight in the band, I can choose a 30FF to get the same fit in the cup with a band that fits more comfortably. In extreme cases, a 32F or 34E would be a suitable fit for me also, but definitely risks being too big in the band. If a 26GG existed, this would also be a sister size if I were to size down in the band.

One question a lot of customers ask is ‘if I change my cup size, do I need to change my band size too?’ There is usually no need to change your band size unless it's too loose or too tight. If you are looking at a dramatic cup size change (like, from a B cup to a G cup), then you may in this circumstance need to amend the band size as the too-small cup is likely to have been disguising how the band actually fits. I know, it's confusing, but if you're just looking at a cup size or two difference then you can stick with your normal band size.

It can be tempting to use your sister size to buy that gorgeous bra that doesn't come in your true size, but try to avoid making a habit of this. A bra that is too loose in the band won’t offer the correct support resulting in achy shoulders and a sore back, while a bra that is too tight in the band can cause some serious pain and bruising (although you can use a bra extender to help with this if needed).

I hope that this has helped with some of the confusion that can come with sister sizing and how it works, but as always let me know if you have any further questions in the comments below, or feel free to tweet us or message us on Facebook.

LJ x

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