What are sister sizes for bras?


You may have heard the saying “boobs are sisters rather than twins” meaning that our boobs are rarely identical, but did you know that ‘the girls’ can have sister sizes, too?

Sister sizes can be useful when tweaking the fit of your bra and if you’re struggling to find the size that you need in a bra that you love.

Pretty cool right, but what does sister bra size mean exactly? Put simply, sister bra sizes are bra sizes that share the same cup volume despite having different back sizes and cup letters. The beauty of sister sizing means that you can fit into several bra sizes that have the same amount of space in the cups (and hold the same amount of boob!) even though the numbers and letters are different.

In other words, sister bra sizes are all the equivalent bra sizes that will fit ‘one size’ of boobs.

You may well be smaller in the back and bigger in the cup than you think...

We hear from lots of women who have been told that they are a certain cup size, but cup size is only half of the picture.

Because the availability of bras in bigger cup sizes can be limited outside of the UK, lots of women have got used to wearing bras that are too big in the back and too small in the cup. This can cause your bra to ride up at the back or dig in to your shoulders (ouch!). Although the cups might fit your boobs well, the back band isn’t firm enough to support your boobs, so a sister bra size with a smaller back band and bigger cup size will be more supportive.

:Kelly, Bravissimo girl

"I had always thought I was too small for Bravissimo but my previous size 36C is actually 32E! I tried styles I could only dream of wearing before and it was just amazing to see the range of underwear and swimwear that will fit me properly. Thank you Bravissimo!"

“But I’m a DDD cup, not a G cup!”

As a guide, a 28 back is a US size 4, 30 back is a size 6, 32 is a size 8, 34 a size 10, 36 a size 12 and 38 a size 14, so if you’re a US size 8 and wearing a 38DDD – you might feel more comfortable and supported in the UK bra sister size 32G. Choosing a sister size that’s smaller in the back can make a huge difference not only to the way you look but the way you feel too!

But how do sister sizes work when cup sizes are so different? In the UK where we have been fitting women for 25 years, our most popular sizes are FF, G and GG cups. These cup sizes may sound big if you’ve always thought that the biggest size you could get was a DD cup, but they really aren’t – they are totally normal.

:Hannah, Bravissimo girl

"I was amazed to find out I was a 36H when I’d been wearing a 38E! What surprised me even more was how much more confident I felt wearing a bra that fitted me!"

Do sister sizes really work?

Are bra sister sizes actually accurate? They certainly are! Sister sizes work on the principal of ratios – it’s not sorcery – it’s a bit of math! So 34FF is the same as both 32G and 36F, because you’re adjusting the cup to back ratio. Wear any of the three and A-BRA-CAD-A-BRA – all three have the same amount space in the cups. (OK, we agree it is a bit magic!).

Calculate your sister size with our conversion chart

Our sister cup size chart shows all the sizes with the same cup volume in the same row (going across). Each row is a family of sister sizes. So, if you’re currently wearing a 40DD, your sister sizes are 38E, 36F, 34FF, 32G, 30GG and 28H.
The chart uses UK sizing (just like our bras). If you’re not sure what your UK bra size is, use our UK to US bra size converter to find out!

This sister size chart uses UK sizing (just like our bras). If you're not sure what your UK bra size is, use our UK to US bra size converter to find out!


What's my sister size if I don't have a chart?

If you don’t have the chart handy, you can work out your sister bra size by going down a band size (or two, or three!) and up a cup size (or two, or three!) and vice versa.

We know that it may sound a bit tricky, but we promise you don’t have to be a professor at Bra-vard to work out your sister size - it’s really easy when you know how!

Here’s a little trick we like to use!

If you’re going ↑ cup size, you need to go ↓ back size = the same cup volume

If you’re going ↓ cup size, you need to go ↑ back size = the same cup volume

Join the sisterhood!

If you have any questions about bra sister sizes or would like any more help to find your feel-good fit, please get in touch with our friendly fitters over the phone (it’s toll free!), via Live Chat on the website, or why not visit our virtual fitting room from the comfort of your own home? You can book an online video fitting here.

We’d love to help you to find a size that you feel amazing in and if you’ve recently had a fitting that has left you with a (well-supported!) spring in your step please drop by our socials and let us know – we’d love to hear from you!


:Hannah, Bravissimo girl

"I had always dreamed of having a great fitting bra and after my fitting, I discovered I am a 28F - not a 32D like I'd been wearing for years! I feel like this was a life-changing experience for me. What a difference a good bra can make!"

read our guides below, or take our bra fit quiz to find your feel-good fit!