What are sister sizes for bras?
You might be surprised to learn that there’s more than one size option when it comes to finding your perfect bra fit!
Sister sizes for bras refer to alternative bra sizes that have the same cup volume – even though their back sizes and cup letters differ. This means, if your usual bra size isn’t quite right in a particular style or brand, you can try a sister size to find the best bra fit, comfort and support for you.
Do sister sizes really work?
Are bra sister sizes accurate? Absolutely! We might be bra wizards, but there’s no sorcery here. Sister sizes work on the principle of maintaining the same cup volume while adjusting the band size, or what’s called the cup-to-band ratio.
For example, 34FF is the same size in the cups as both 32G and 36F. While the back band size changes, the amount of space in the cups stays the same, helping you find a fit that suits your shape better if your usual size feels too tight, too loose, or too uncomfortable.
And don’t let the letters throw you off! Remember, the cup volume stays the same; it’s just the band that changes. Choosing a smaller back band can feel more supportive and comfortable, even if the cup letter seems bigger. In fact, larger cup sizes like FF, G and GG cups are our most popular sizes.
How to find my sister size
Finding your sister size is easier than you think. Using our bra sister size chart, you’ll see that that all the sizes with the same cup volume are arranged horizontally in rows – these rows are your ‘sister size families’. So, if you’re currently wearing a 40DD, your sister sizes are 38E, 36F, 34FF, 32G and 30GG.
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.
Simply put:
If you’re going up a cup size, you need to go down a back size
If you’re going down a cup size, you need to go up a back size
When should I try a sister size?
Try sister sizing when your bra band feels a little too tight or too loose, your cups aren’t fitting quite right, or you want to try different bra style or brand (especially if your usual style is out of stock!).
And if one boob is slightly bigger than the other (let’s be honest, whose isn’t?), or you’re stuck between two sizes, sister sizes make finding the perfect fit even easier!
Need help?
If you have any questions about bra sister sizes or would like help finding your perfect fit, then we’d love to hear from you! You can call us (it’s toll free!), Live Chat with us, or why not visit our virtual fitting room and enjoy everything we have to offer from the comfort of your own home?
FAQs
Each bra size is surrounded by a family of sister sizes. They all have the same cup volume, but the band and cup letter change. For example, 40DD has the same cup volume as 38E, 36FF, 32G and 30GG.
Sister sizes work on a simple ratio: if you go up a cup size, go down a back size – and vice versa. As long as the cup volume stays the same, adjusting the cup-to-band ratio lets you try out different sizes for your perfect fit.
Sister sizes can be really useful if you’re between sizes, or if your usual band feels a little too tight or too loose. They’re also handy when shopping – if your go-to size isn’t available, or you’d like to try a different style or brand.
Maybe! If your usual band size doesn't feel quite right but your cups fit well, it could be time to try a sister size. They’re also super handy when your normal size isn’t in stock (we’ve all been there).
Not always. While the cup volume stays the same, a tighter or looser back band can change how a bra feels and supports you. Remember that the back band does most of the support, so always make sure it fits comfortably.
This is a good question and really depends on what you need. If you want a firmer fit, try going down a band size and up a cup. If your bra feels too tight, try going up a band size and down a cup.
Our expert fitters are always on hand to help you find your best fit and explain how sister sizes can work for you. Why not get in touch with us today – we'd love to hear from you!