How bra sizing works

We have spoken to many amazing pregnant women and moms, and 70% of them said they are always confused with their bra size and how bra sizing works! Finding the correct bra size is crucial for the right support comfortable fit, even more so during pregnancy. That's why we decided it was time to explain bra sizing once and for all!

No universal bra sizing

There are no strict, international industry standards for bra sizing. The rules differ from one country to another and even among brands and manufacturers. These discrepancies probably won't change any time soon. And even if a universal bra size chart was created, it's unlikely every lingerie brand would adapt it to their products because they care more about keeping their long-term customers undisturbed than simplifying and harmonizing the state of the industry in general.

So, all we can say is that traditional bra sizes are a combination of letters and numbers. But now, we see a lot of brands using the XS/S/M/L/XL sizing system. This simplification may work for some of you, alas the lack of accuracy doesn’t help for most of us!

How to determine your US bra size

Of course, many lingerie boutiques and department store employee can take your measurements, but in this era of online shopping, knowing how to properly take your own measurements is very convenient.

All you need are your boobs and a measuring tape!

Step 1: Take your bust measurement

Wrap the measuring tape around the fullest part of your bust at nipple level. Make sure the tape is parallel to the ground and isn’t too tight. Round to the nearest whole number.

woman measuring her bust wearing black Infiniti Bra and using white tape measure

Step 2: Determine your band size

Wrap the measuring tape around your ribcage, just under your bust where the bra band would naturally sit. The measuring tape should be parallel to the ground and snug. Round to the nearest whole number.

Then, things can get weird... If this number is even, add four inches; if the number is odd, add five inches. Your band size is the sum of this calculation.

So, if you measured 27 inches, your band size would be 32; if you measured 32 inches, your band size would be 36.

Historically, this has been standard practice and no one is really sure why. Some say the “plus-four” method was created in the 1950s to allow breathing room when bras were made out of un-stretchy material like silk and satin, something which in 2021 is completely absurd as all bras have elastic and stretch in them. Some manufacturers have started to give up on this rule, so it’s very important to consult each fit guide to determine whether you’ll need to do this addition.

Step 3: Determine your cup size

Your cup size is represented by the letter portion of your bra size. To find this, you’ll need your bust measurement and your band size. Subtract your band size from your bust measurement.

The traditional U.S sizing system says to go up one cup size per inch of difference between your bust measurement and your band measurement, so a 34-inch band with a 35-inch bust would be an A-cup and your bra size would be 34A, a 34-inch band with a 36-inch bust would be a B cup and your bra size would be a 34B, and so on.

Difference between bust and band measurement Cup Volume
Less than one inch A
Less than two inches B
Less than three inches C
Less than four inches D
Less than five inches DD

Bear in mind that this is just a guide for you to find your best fit. As we said before, bra sizing isn’t standardized, each brand determines their own fit and your particular breast shape may also affect the way certain bras fit and feel. Use this as a starting point, but don’t be surprised if you have to try a few bras to find the right fit.

The importance of sister sizing

Now that you know how to find your bra size, let’s explain the concept of sister sizing! Sister sizes are groups of bra sizes that are equivalents by cup volume and essentially offer the same fit. UH, what?? Yes, it is very confusing, but knowing your sister size can be helpful when adjusting your bra size, for example you may notice your bra cups fit okay but your back band is a little loose.

In this case, you’ll want to keep the cup volume of your current bra but get a smaller band. And this is when the chart comes in handy! Your sister sizes are the ones listed in the same row going across as your current bra size. For example, if you’re a 34B, your sister sizes are 30D, 32C, and 36A.

If your cups fit okay but your back band is a little too loose, you could try a smaller band size as well as a larger cup size to maintain the same cup volume. In this case, if you are in a 34B, you could try a 32C. Going down in the band size alone will result in the cup size being too small. Though 32B sounds like it has the same cup volume as 34B, it’s actually one cup size smaller.

Because of the way sister sizing works, you can see that A = small cups and DD = huge cups is a total myth!!! A 30DD bra has the same cup volume as a 38A bra!

You now know everything about bra sizing! Finally!

Helpful hint: because our bodies are constantly changing, your bra size may change regularly, so you should plan to get a bra fitting at least once a year. Depending where you are in your cycle, if you have gained/lost weight, or if you’ve just given birth, your size might seem way off from the last time you measured. This is completely normal so it’s good to familiarize yourself with what your natural range of measurement is!