How bra sister sizes help you find your perfect fit

Have you ever bought gorgeous lingerie in your usual bra size, only to find the darn thing doesn’t fit you properly when you try it on? Don’t worry. It’s not you, it’s the bra!

Yes, you may be exactly a 38 in the underband. And yes, your go-to T-shirt plunge may be a DD bra size. But band and cup sizes aren’t the only thing to take into consideration when it comes to getting the bra you love to fit like a glove.

Every woman’s breasts will have a slightly different shape and degree of fullness. Their position on the chest will vary, as will the amount of ‘separation’ between them. With all of this to take into consideration, it’s a wonder anyone’s breasts fit neatly within the standard bra sizes on offer.

You also have to take into account the shape of the bra you’re trying to fit into. That cute underwired balconette may look pretty, but if your breasts are full and wide-set, you may find it hard to get the underwire to sit exactly where it should, following the natural crease of the bust.

Thankfully, lingerie manufacturers know this too. They’ve developed a technique that can help when you find a bra you love, but that you can’t quite get to fit your shape perfectly. The technique is known as sister sizing, and it’s one you can easily steal for yourself.

What are sister sizes?

There’s nothing like the support you get from a beloved sister to help you feel your most comfortable. And it’s exactly the same when it comes to your bra. Whether you’re a 34D or a 38F, your normal bra size actually comes from a family of sisters, related bra sizes who share one important quality in common - the volume they hold in the cup.

While dress sizes go up in number the bigger they get, bra sizes don’t work like that. In fact, they’re more like ratios because the volume of the cup size (the amount of breast tissue it can hold) varies with the band size.

Let’s say you’re a 34C and you try on a bra. You find the cup size is fine, but the band’s a bit too tight. If you didn’t know about sister sizes, you’d probably opt to try a 36C next. But because you’ve gone up a band size, you’ve also gone up a cup size without even realising it!

Instead, you should try a 36B, because while there’s more room in the band, the cup volume is exactly the same. In this way, sister sizes are far more accurate.

To give you another example, if you’re a 38C, your bra belongs to this family: 30F, 32E, 34DD, 36D, 38C (your size), 40 B. So your closest sister sizes are actually 36D and 40B, because while the band may vary, the cup volume is exactly the same. You’d also find that 30F, 32E and 34DD would fit you in the cups, too. So a 38C has the same bra cup size as a 30F. Pretty mind-blowing, right?

Why do bra cup sizes change with the underband?

It does seem a bit weird that the cup size changes with the underband, but not when you understand how bra sizes work.

You cup size is the difference between your underbust and overbust measurement, so it shows how far your breasts ‘stick out’ from your body. If the difference is 2.5cm, you’re an A cup. If the difference is 5cm, you’re a B and so on.

But how much your breasts stick out doesn’t say anything about how wide they are. So someone who is a 40C will have much more breast tissue overall than someone who’s a 32C, so they need bigger cups that can hold more volume.

How do sister sizes work?

Take a look at the bra size chart and find your normal size (if you’re not sure which size you are, find out how to measure your bra size).

Then, keeping on the same row, look at the sizes immediately to the left and right of your own bra size. These are your sister sizes.

If a bra feels too tight in the underband, you’ll need to move one space to the right. Doing that means you’ll go up one band size while going one size down in the cups.

It works the same in reverse. If the bra is too loose in the underband, you’ll need to move one space to the left. Doing that means you’ll go down a band size but up one size in the cup.

Are sister sizes accurate?

Sister sizes do work well, but it’s worth remembering that there are limits to what you can achieve. Sister sizing one up or one down is fine, and you might even be able to push it to two. But beyond that, the proportions of the bra will have changed so much that the bra probably won’t be a good fit.

When else might I need to use my sister sizes?

Here at Pour Moi, we’re known for our huge range of band and cup sizes, but that’s not always the case elsewhere. Luckily, sister sizing gives you lots more options.

Say you’re normally a DD bra size but the gorgeous plunge you’ve got your sights on only comes in a D. Using your sister size you could just go up to a large band, and down to a D and still get a good fit.

  • Finding your perfect bra is easy at Pour Moi. From sensuous lace bralettes to the most comfortable everyday options, you’ll get to choose from an incredible range of styles. But there are also such a huge range of sizes on offer too, from an A to a J cup and up to a 44 back, that you may not have to call on the help of your sisters after all...
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