Everything You Need to Know About The Bra Lab’s Sizing System

How to Find the Perfect Fit With The Bra Lab

To make finding your perfect fit as easy as possible, we’ve introduced a number-based sizing system.

Your cup size is a number (1,2,3...9) and your back band size is either Small, Medium, Large or XL. All shoulder straps are one size and fully adjustable.

 Is this sending your head into a spin? Don’t worry – we’ve got a converter that’ll convert your traditional size or measurements for you instantly. 

So why do you have to convert my size, you ask?

Our key bra components at The Bra Lab are the cup and the back band. Unlike traditional bras, these come as separate pieces. If we could use the traditional bra sizing system (eg 32A, 42DD, etc.) we would! We simply can’t do this as our key bra pieces are separate and they don’t change. 

Bra Lab size

Why Number Sizes?

At the Bra Lab we used to use letters. However we found that customers were tempted to buy their traditional 'cup' size. For example, a customer who is (for example) a 32D would be tempted to purchase the Bra Lab D cup with the medium back band. However, what she actually needed was a B cup. Because all cup sizes are relative to the band... (more on that in a minute).

How Does The Bra Lab Sizing Compare to Traditional Sizing?

In traditional bra sizes, your band size and cup size are all relative. This is very important to understand. To give you an example, a 30D is actually a very different breast volume and size to a 40D. The cup on a 40D is much larger. The breasts of this person are much larger. So you can’t say ‘I’m a D cup’ without telling us your full size (e.g. 30D, 32D, etc.). Simply saying you’re a D cup doesn’t tell us how big your boobs are. Truth.

With the Bra Lab’s sizing, you need to find two separate sizes - a cup size to fit your breasts comfortably and a back band size to support it. Telling us your full traditional bra size will enable us to know the size/volume of your breasts and so we can give you your Bra Lab cup size. Your band size also provides your Bra Lab back band size - but it’s better to look at these numbers together and convert them in our size converter rather than just looking at just one half of the equation. 

Our number system also helps keep new shoppers from making sizing mistakes. With our previous sizing system, women often assumed their cup size would be the same in traditional sizing and Bra Lab sizing and would be disappointed when the fit wasn’t quite right. With our number system, there’s no confusion – it’s now a unique sizing process!


What Else Should I Keep in Mind When Choosing My Size? 

Studies that show over 60% of women are wearing the wrong bra size. Also be aware that if you haven’t been fitted for a bra in many years, it’s likely your bras have stretched and you’re a little bigger than the size on your bra tag. If you have had babies and have been breastfeeding, you might find your breasts have lost some of their volume, making you a smaller size now than you were previously. And in some cases, the reverse can happen. It’s always beneficial to have a proper bra fitting or you can also send us your bust and underbust measurements to shop@thebralab.com and we can help you. 

So How Do I Get Started?

Use the bra builder to convert your traditional size and find your bra lab size.


Refer to the sizing chart on our cups page.

Frequently Asked Questions 

Why don’t you just use regular sizing?

We simply can’t use conventional sizing, as our cups and back bands are sold separately. In traditional bra sizing, cup volume increases as your band size increases. For example, a 30C is a much smaller cup than a 44C - even though they are both Cs!

With the Bra Lab, you need to find your unique cup size. Then separately, you need to get your band size.


Is this sister sizing?

No, we don’t use sister sizing. Sister sizing refers to someone being able to fit into closely related sizes – for example, a sister size to a 32B is a 30C. These two sizes are very similar in terms of cup volume and often people hover between two sizes or can fit their sister size depending on the bra brand.

At The Bra Lab, we use a unique sizing system that gives one size for your breasts and one size for your back.


I’m getting two different sizes depending on whether I input my traditional bra size or my measurements into the sizing converter. Which one is correct?

If you get very different sizes when you do this, opt for the size that your traditional bra converts to. We find that many customers fluctuate with their measurements or don’t measure correctly. Just make sure the traditional bra size you’ve input is a recently purchased one and you haven’t stretched it over time.


I tried a cup size that was too small so I exchanged for the size up but now that cup is too big. What do I do?

This occasionally can happen, and we suggest playing around with your back band size. So if you went up in the cup size maybe you want to go down in the back band size.

Your breast shape may also not be an ideal fit for the cup itself. We have designed the Angelina as a molded contour cup that will work with most breast shapes. If it doesn’t work for you, please stay tuned for our wirefree Roseanna and other bra styles coming out in the next few months. A different style may be just what you need!


Any more questions?

We're here to help. Leave a comment below or email us at shop@thebralab.com. You can also DM us on our socials or message us on our website. All messages go direct to our business owners and we respond within a day (usually a lot less). 


The Bra Lab team

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