Sizing Guide

Sister Sizes

Your current bra size has "Sister Sizes" - alternate sizes where the cup volume stays the same even though the band size and cup letter changes. This means that even if we don't have your bra size in stock, you can still order a bra that fits you, by choosing the corresponding Sister Size.
 

Find your Sister Size

Your bra size has two Sister Sizes going "up" and "down". "Up" means that the Sister Size is one actual size larger and one letter name smaller than your current bra size. "Down" means the opposite - one actual size smaller and one letter name larger. For example, if you wear a 38B, your Sister Size Up would be 40A, and your Sister Size Down would be 36C. You can wear either one of the Sister Sizes and still have the same cup volume as 38B! The only difference is that you'd probably need to use the furthest or the tightest hook of the bra, instead of the middle one.

Our Sister Sizes Guide

The chart below will help you find your Sister Sizes. Just look for your actual bra size in the middle column.

Sizes in pink are stocked by BackStory.

 
30 Band Size
Sister Size DOWN Current Bra Size Sister Size UP
28DD 30D
32C
28DDD 30DD
32D
    
32 Band Size
Sister Size DOWN Current Bra Size Sister Size UP
30D
32C
34B
30DD
32D
34C
30DDD 32DD
34D
    
34 Band Size
Sister Size DOWN Current Bra Size Sister Size UP
32B
34A 36AA
32C
34B
36A
32D
34C
36B
32DD
34D
36C
32DDD 34DD
36D
    
36 Band Size
Sister Size DOWN Current Bra Size Sister Size UP
34B
36A 38AA
34C
36B
38A
34D
36C
38B
34DD
36D
38C
34DDD 36DD
38D
     
38 Band Size
Sister Size DOWN Current Bra Size Sister Size UP
36B
38A 40AA
36C
38B
40A
36D
38C
40B
36DD
38D
40C
    
40 Band Size
Sister Size DOWN Current Bra Size Sister Size UP
38C
40B
42A
38D
40C
42B

 

For most people, shopping for clothes is pretty straightforward, but when it comes to shopping for bras, it's not always so simple. There are different factors that play, for example, your breast shape, your cup size, and your band size. Unfortunately, they don't always match with a number and letter, but this is where bra sister sizes come in. 

What Are Bra Sister Sizes? 

Bra sister sizes are groups of bra size equivalents related by volume. For example, the 36F cup is in the same family as the 38E, 40D, and 42C. Sister bra sizes are the sizes that surround your bra size. As the size of bras offer something different, either due to the manufacturer, the style, the brand, or even the country it comes from, it's important to know about sister bra sizes so you can try other sizes if the one you have doesn't match your current size properly. You may have learned that if you had a bra size and it fits okay, then you would have to settle on that, but you may have also thought your size would be two different cups. It's important to remember that bra sizes are more like ratios. And the volume of the cup size will vary with the size of the band. So let's delve further into this. 

Why Should You Know Your Bra Sister Size?

It's important to know your bra sister size. If you have issues trying to find a bra that fits your unique shape, you know it's never easy. And it's important to remember that it's not to do with you; it's the bra itself. Identifying your sister size can be helpful because if you find that the closest bra that suits you doesn't fit properly either because of the band or the cup, knowing your bra sister size will make a massive difference.

Should You Know Your Cup Size? 

Definitely! If you are trying to calculate your sister size, you have to remember that a sister size is a size that has the same cup capacity but different size of the band. So for example, if you are a 32C, you could also wear a 34B or go down to a 30D, depending on if you want a looser or tighter fitting bra. It's important to know your true cup size before you start to work out your sister sizes.

How Do I Calculate My Bra Sister Size?

The rule is pretty straightforward:

  • If your band is too tight, you go up a band size, but you go down one cup size

  • If the band is too loose, you go down a band size but you go up a cup size.

Here are a few examples:

  • If your current bra size is 36A and you want to go down a sister size,it would be 34B. If you want a sister size up, it would be 38AA.

  • If your current bra size is 20D, if you were to go down a sister size it would be 26DD, and if you were to go a sister size up,it would be 30C.

  • If your current bra size is 46DDD, if you go a sister size down, it is 44F, and if you go a sister size up it is 48DD.

A Few Extra Things to Consider

It's great when you find one that fits perfectly, but there can be a few scenarios where it may not feel as perfect as it could. So here are a couple of things to consider in certain scenarios:

Your Cup or Bands Do Not Fit

In this situation, go down a band size if both are too big, or if they are too small, you go up a band size. But do not worry about changing your cup size because it will adjust naturally with your band. 

The Cup Fits, but the Band Doesn't

If this is an issue, you need to adjust the band and the cup size. So if you go up a band size, you need to go down a cup size.

The Band Fits, but the Cups Do Not

If the cups feel small you should go up a cup size. But if the cups feel big, you go down a cup size.

Finding the perfect bra size is going to make a big difference to your level of comfort. It's important to remember that over time, bras will loosen and lose their shape so the underband could feel loose if you are wearing an old bra. If you go for a newer bra, it will be firmer than you are used to. You have to remember every brand and style will fit differently, so when you're looking for the perfect one for you, you should try a variety. But if you don't know your sister size, this could make a massive difference to your level of comfort.