What Determines Bra Size?


Ever wondered why your breasts are the size and shape they are? It’s a common question, in no small part fuelled by the idea that there’s an “ideal” size and shape for women’s breasts.

While that could easily lead us on a long diatribe about unrealistic standards of beauty that haunt women (not to mention lacking diversity too), instead we’ll do our best to address the question at hand.

How is Breast Size Measured?

Before we can really talk about breast size, we should first mention how it’s measured. Breast size is usually stated in terms of size of bra. For a deeper dive on measuring bra sizes, click here. We’ll summarize quickly below. However, please note that this method of measuring bra sizes may not be 100% reliable when it comes to bra shopping (for that, we recommend a fitting).

First, Measure Your Bra Band Size

Use a (flexible, sewing) measuring tape to measure around your torso directly under your bust, where a bra band would sit. 

Remember the idea is that you get most of your support from the underwire or band rather than all the support coming from your shoulders (ouch!) So, the tape should be level and snug beneath your bust, parallel to the ground. The measurement in inches is your “band size”, according to this method. 

Let’s say you measured 34, this is what starts your bra size! 

If the number you measure is odd or a fraction, round up to the nearest even number and then add two. E.g. if you measure 33 or 33.5, round up to 34.

Then, Measure Your Bust Size

The second measurement you need to take doesn’t appear in the bra size, but it is used to calculate the cup size. 

So, now, using the same measuring tape, measure the fullest part of your bust. If your breasts are fuller at the bottom, it may be easier to lean forward from the waist when you measure the fullest part. You’ll want to try to have the tape run parallel to where it sat when you measured your band.

Let’s say you measured 38 here.

Finally, Calculate Your Cup Size

Now, to calculate your cup size, you’ll subtract your band size from your bust size. Each inch represents a cup size, as shown from in the table below.

For our example, that’s 38 minus 34, which is 4.

4 inches is a D cup.

So the right bra right size according to this method is 34D.

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Bust Size minus Band Size (inches)

Cup Sizes

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What Determines Breast Sizes?

Okay, now you know your breast size, you might wonder WHY? Why is one person a 36D and another a 32E. In a way, this is like wondering why we don’t all have the same size and shape noses… because we’re all different! 

While there are averages (the average American bra size is 34DD) we all know that averages are just that. And for every average bust size there’ll be many on either side, to varying degrees — boobs that are big, books that are small and everything in between.

So what causes such extreme differences in bust size? What causes bigger breasts? The answer is really in your genes.


Yep, you can mostly blame (or thank) your genes for both the size and shape of your breasts, and even the composition of your breast tissue. 

According to Nature, “Breast size is a highly heritable trait. A twin study previously estimated the heritability of bra cup size to be 56%8. Several genome-wide association studies have also identified common genetic variants associated with breast size.”


Another major factor that can be both nature and nurture is body weight. Breasts are complex body parts but they are composed significantly of adipose fat. This can mean that individuals with higher body fat will often have larger breasts.

What About Nutrition?

There is speculation about whether certain diets contribute to larger breast size. The American Journal of Clinical Nutrition studied connections between dairy intake and breast size, for example. And you’ll find countless Reddit threads citing supplements and diets that can increase or decrease breast size. 

In our research, there was no prevailing data to consistently claim a connection between certain food groups or supplements and breast size. If you consider taking a supplement or changing your diet with hopes of enhancing your breast size, please do consult with your doctor first. 

What Can Cause Breast Size to Change?

While breast size is mostly determined by your genes, that doesn’t mean it stays the same throughout your life. Indeed, most women experience many changes. Here are some of the key causes of changes to your breast size (and shape).

Age: Puberty to Menopause

From puberty through maturity and into menopause, our bodies go through many changes. These developmental stages are often ushered in by hormonal changes and hormonal changes can definitely affect your boobs. 

Menstrual Cycle

Hormones also play a major role in your menstrual cycle and these can cause monthly changes to your breast size. Many women especially notice breast swelling and tenderness just before their period as part of their premenstrual symptoms (PMS) and/or during ovulation, around the middle of their cycle.

Hormonal Birth Control

Hormonal birth control is used to prevent unwanted pregnancy but the hormones in play can also cause fluid retention and breast swelling in some women. These effects are often stronger in the first few months of taking a new birth control pill.


During pregnancy, your breasts may double or triple in size. According to the Mayo Clinic,

“One of the first signs of pregnancy is an increase in breast size. As early as two weeks after conception, your breasts start to grow and change in preparation for producing milk. Stimulated by the hormones estrogen and progesterone, the milk-producing glands inside your breasts get bigger and fatty tissue increases slightly.

By the end of your first trimester, your breasts and nipples will be noticeably larger, and they may keep growing throughout your pregnancy. Breast enlargement accounts for at least a pound of the weight you gain while pregnant.”

Your breasts may remain enlarged for a while after birth.

Weight Gain or Loss

Weight fluctuation is one of the many reasons it’s recommended you go for a bra fitting regularly (every year or so). This is because the fat component of a woman’s breasts can make it a body area she’ll easily lose or gain weight.

However, it’s worth noting that it is difficult-to-impossible to target fat gain or loss to a specific body area. So if you lose or gain weight it will be distributed throughout your body, affecting other areas as well as breasts.


Among the many health risks associated with smoking is a loss in skin elasticity, which can affect the delicate skin and breast tissue. This can cause breasts to sag. Some women perceive this as a change in size, though it’s really more related to shape unless accompanied by other changes.

Breast Augmentation

Of course, one of the more obvious ways to change breast size is to go under the knife. Whether it’s a breast augmentation or reduction, this will cause a more extreme and often permanent change to your breast size. If you are considering a breast augmentation, please seek out medical advice.

No Matter What Your Size, You Deserve a Bra That Fits

Wearing a bra won’t affect the size of your breasts (unless you add external padding). But while you’re wearing one, your bra can help you maintain your desired silhouette. To make sure your bra feels comfortable and gives you adequate support, consider getting fitted properly.

The wrong size or fit of bra will ruin your day. You’ll feel uncomfortable, maybe even sore, and just want to get home so you can rip the damn thing off. 

At Knix, we really wanted to help more women avoid this experience altogether. As an online retailer, we know it can be especially daunting to make a bra purchase online. That’s why we developed our virtual fit program. Learn more about it today and book your appointment!

What to Expect from a Knix Virtual Fitting:

  • Book your virtual fitting here
  • We’ll connect you with one of our expert Knix team members for a 1-on-1 fitting session over video chat. 
  • For your bra fitting, we recommend you wear a bra with little or no padding and a fitted t-shirt to make sure we get an accurate measurement. You’ll also need either a) a soft measuring tape or b) a piece of string or cord and a tape measure or ruler.
  • With different styles and sizes for 30A to 42G in the world’s most comfortable and supportive wireless bras, our fit experts can help you get into something that you will love to wear every single day.