Sister Sizing Explained – Find Your Equivalent Bra Size

Have you ever found the perfect bra online, only to discover your size is sold out? Or tried on a bra that fits perfectly in the cup but feels too tight in the band? That's where sister sizing comes to the rescue.

Sister sizes are bra sizes that hold the same cup volume but have different band and cup letter combinations. If you understand how sister sizes work, you can find a bra that fits even when your usual size isn't available.

This guide will explain everything you need to know about sister sizing – how it works, when to use it, and how to find your perfect sister size match.

What Are Sister Sizes?

Sister sizes are bra sizes that have the same cup volume but different band sizes and cup letters. The cup volume stays exactly the same – only the band and cup letter change.

For example, 34C, 32D, and 36B are all sister sizes. They all hold the same amount of breast tissue, but the band is different:

• 34C – your standard size
• 32D – smaller band, larger cup (sister size down)
• 36B – larger band, smaller cup (sister size up)

The rule is simple: Band – 2 = Cup + 1 (tighter band, bigger cup) and Band + 2 = Cup – 1 (looser band, smaller cup).

Complete Sister Size Chart

Use this chart to find your sister sizes. Find your size in the left column, then look across to see your sister size options.

* Sister size rule: Band – 2 = Cup + 1 (tighter band). Band + 2 = Cup – 1 (looser band).

Use our Sister Size Calculator to find your matches instantly.

When to Use Sister Sizes

There are several situations where sister sizing can save the day:

• Your size is sold out – Sister sizes give you more options to find a bra that fits.
• Different brands fit differently – Some brands run small or large in the band. A sister size may fit better in a specific brand.
• The band feels too tight or too loose – If the band is uncomfortable but the cup fits, try a sister size.
• You're between band sizes – If you're between a 34 and 36 band, sister sizes offer a middle ground.
• During pregnancy or nursing – Your body changes, and sister sizes give you flexibility.

How to Calculate Sister Sizes

The rule is simple:

Example:

• Your size: 34C
• Sister size (tighter band): 34 – 2 = 32, C + 1 = D → 32D
• Sister size (looser band): 34 + 2 = 36, C – 1 = B → 36B

How Sister Sizing Changes by Band

As your band size changes, the cup letter changes too. Here's a quick example for each band size.

When Sister Sizing Doesn't Work

While sister sizing is helpful, it's not always the perfect solution. Here are some situations where sister sizing might not work:

• If the cup shape is different – Sister sizes maintain volume but not shape. Some styles may not work with the different band/cup combination.
• If you go beyond 2 band sizes – Generally, stick to within 1-2 band sizes of your original. Going too far changes the fit significantly.
• If the bra is a specific style – Some styles like push-ups or sports bras fit differently and may not sister-size well.
• If you have a very small or very large cup – In extreme cup sizes, sister sizing may not give a good fit.

Tips for Sister Sizing Success

• Always try on sister sizes – Don't assume they'll fit the same. Try them on to see which feels best.
• Check the band – Your band should fit firmly on the loosest hook when new.
• Check the cups – Make sure there's no spillage or gaping in the cups.
• Check the gore – The center piece should lie flat against your chest.
• Check the straps – Straps should be comfortable and not dig in.

Frequently Asked Questions