If you have ever tried on two bras in the same size and thought why does one fit perfectly and the other feel completely wrong — you are not alone. This is where sister sizing comes in, and once you understand it, bra shopping becomes so much easier.

Sister sizing is one of the most helpful tools in lingerie fitting, especially when your usual size does not feel quite right or is not available.

Let’s break it down in a simple way.

What Is Sister Sizing

Sister sizes are bra sizes that have the same cup volume but different band sizes. When you change the band size, the cup letter must change too in order to keep the same overall breast volume.

In short
Smaller band means bigger cup letter
Bigger band means smaller cup letter

The cup volume stays the same even though the size label changes.

How Sister Sizing Works

If your current bra size is 34C, here is how sister sizing works.

If you go down a band size to a 32
You need to go up a cup size
Your sister size becomes 32D

If you go up a band size to a 36
You need to go down a cup size
Your sister size becomes 36B

Same boob volume
Different band fit

Easy Sister Size Examples

Here are a few common sister size chains to help you visualise it.

30D becomes 32C then 34B then 36A
34C becomes 32D or 36B
36D becomes 34DD or 38C or 40B

This is why the letter alone does not tell the full story — cup size is always relative to band size.

Why Sister Sizing Is So Helpful

When the Band Feels Wrong

If your cups fit perfectly but the band feels too tight, try a sister size with a larger band and smaller cup letter.

If the band feels too loose but the cups fit, try a sister size with a smaller band and larger cup letter.

When Shopping Different Brands

Every brand fits slightly differently. Sister sizing gives you flexibility to adjust your fit when switching brands or styles without guessing blindly.

When Your Size Is Out of Stock

If your exact size is unavailable, a sister size can often give you a very similar fit.

Things to Keep in Mind When Sister Sizing

Even though sister sizes share the same cup volume, they will not feel identical.

The band will feel firmer or looser
Strap placement may change slightly
Overall support can feel different

Always check that
The band sits snug and secure
The cups fully encase the breast
There is no spilling or gaping
The straps feel comfortable once adjusted

Comfort is always more important than the number on the label.

Sister Sizing Works Best With the Right Foundation

Sister sizing is most effective when your starting size is close to correct. That is why professional bra fittings are such a game changer.

At Tutti Rouge, we always recommend choosing the size that feels best on your body, even if it is different from what you expected.

Your body is not the problem.
The bra just needs adjusting.

Why Sister Sizing Empowers You

Understanding sister sizing puts you in control. Instead of feeling frustrated when a bra does not fit, you know how to adapt and find a better option.

It helps you shop smarter
Feel more confident
And enjoy lingerie that actually supports you

And that is what we are all about.

Find Your Perfect Fit with Tutti Rouge

Our bras and bralettes are designed with real curves in mind, and we are always here to help you find your best fit.

Whether you need help with sister sizing, style advice, or a professional fitting, we have got you covered.

👉 Explore Tutti Rouge Bras
👉 Shop Bralettes and Supportive Styles
👉 Book a Tutti Bra Fitting