If you’ve ever shopped for clothes, you’ll know that when it comes to sizing it’s a (relatively) straightforward process. Generally speaking, the higher the number/letter on the garment, the bigger is it. 

It’s probably worth adding a caveat that what brand A sees as a size 12 pair of trousers is likely different to brand B. However, in general, a size 12 is going to be smaller than a size 14, etc.

Bra sizing, on the other hand, is not quite as simple. 

When looking for a new bra, you need to consider factors such as your unique breast shape, cup size and band size — all of which rarely match neatly with a single number and letter. 

This is where sister sizes come in – knowing this can help you find your perfect fit.

What is it? 

Sister sizes are groups of bra sizes that are all related to each other by cup volume. For example, a 36E cup belongs to the same family as a 38D, a 32FF, and a 30G. 

Stick with us — we’ll explain.

It’s easiest if we think of bra sizes as being more like ratios. The volume of a bra cup (or how much breast tissue it can hold) will vary with its band size. 

Unfortunately, you can’t just go from a 32E to a 36E if the band of your bra feels too tight. Why? Because an “E” cup size isn’t static, it will change size depending on the band size it’s attached to. 

Someone who is a 38E will have more breast tissue, and generally have a bigger breast, overall vs someone who is a 32E. So they will need a bigger cup. 

So, how does it work? 

When you measure your bra size you get your cup size by subtracting your band measurement from your bust measurement. 

Each inch of difference (between band and cup) indicates an additional cup size (e.g. 6” difference = E cup, 7” difference = F cup, etc). 

So, all this measurement tells you is how far your boobs stick out from your body. It doesn’t say anything about the actual volume of your breast. 

To work out your sister size, think of your bra size as a see-saw. For every back size you go down, you have to go up a cup size. For example, if your measured size is a 34F, your sister sizes are a 32FF or a 36E depending on whether you need your band to be looser or tighter. 

Why is it important? 

In short, it’s essential to make sure you’re getting the proper fit and support from your bra. 

Every style and brand of bra is slightly different in terms of how they fit. Often due to things like the models they use in sampling, the fabric and elastic used and their sizing system. 

Some might be more rigid on the underband, others might be slightly slacker. Knowing your sister sizes means that you can get the perfect fit for you, ensuring you are giving your boobs the right level of support.