Bra Sizing 101 — Understanding Band, Cup, And International Differences

Let’s be honest — bra sizing already feels like a secret code. Numbers. Letters. Double letters. Different rules depending on the country. And somehow everyone else is supposed to just know what fits?

If you’re a trans woman or crossdresser, you may be navigating bras with breast forms, padding, or a body that doesn’t match traditional assumptions — which can add an extra layer of confusion.

Here’s the good news — bra sizing isn’t about fitting into someone else’s idea of a body. It’s about proportions, placement, and how volume sits on your frame. Once you understand the basics, everything gets a lot easier.

Let’s clear up a common misconception right away.

Bra sizing isn’t determined by how your chest looks — it’s determined by the relationship between:

• your rib cage (the band), and

• the volume on your chest (whether that comes from natural tissue, breast forms, or padding)

That’s why bras can work beautifully across a wide range of bodies and presentations.

The number in a bra size (like 34, 38, or 42) is your band size and is based on the measurement around your rib cage but 90% is NOT the same as your rib cage measurement. 

In a well-fitting bra, support is shared. The band and straps each do about half the work — the band stabilizes everything, while the straps help lift and balance the weight comfortably.

Band sizing:

• Uses even numbers
• Most bands range from 28 to 56

The Breast Form Store uses traditional North American bra sizing. This means that the bra band is not the same as what you physically measure, and requires adding several inches to get to the true bra band. This is because traditional North American bra sizing is not based on the underbust, but what someone measures at their armpit when there is naturally occurring breast tissue.

Want to learn more about how North American bra sizing is determined? Watch this episode of Ask Eden (video insert).

Traditional North American bra sizing is determined by a few different steps:

• Taking a physical measurement of your underbust
• Rounding it to the nearest whole number
• Adding the appropriate number of inches to get to your true bra band size
  • If you measure less than 41 inches, add 4 inches
  • If you measure more than 41 inches, add 2 inches
• If your number is odd, round up to the nearest even number to get your true bra band

This is incredibly important for any person who is assigned male at birth (AMAB) or does not have naturally occurring breast tissue (such as breast cancer survivors). This is because without that naturally occurring breast tissue, the measurement taken from the armpit can be misleading, or create an inaccurate fit.

Bra manufacturers understand this, which is why the system of "adding inches" matters!

Cup size is where things get misunderstood — especially for people wearing forms.

Cup size is the difference between your band measurement and your bust measurement — which reflects the total volume on your chest.

A D cup is not a universal size.

For example:

• a 34D and a 38D do not hold the same volume

• cup volume increases as band size increases

This matters when choosing bras or breast forms — because the same volume can fit very differently depending on the band you’re wearing. This is because the person who has a smaller frame size will need less volume to create the shape of a proportionately curvy look than someone with a larger frame.

Sister sizes have the same cup volume but different band sizes.

You can find a sister size by:

• going up a band size and down a cup, or

• going down a band size and up a cup

Example:

• 40D → 42C or 38DD

Sister sizing is incredibly helpful if your band feels too tight or too loose but the cup volume feels right.

There’s no universal sizing system — which means your size can change just by crossing a border.

• Bands in inches, not based on underbust

• Even-numbered bands

• Uses double letters (DD, DDD)

• Bands in centimeters, not based on underbust

• No double letters

• Example: US 32DD ≈ EU 70E

• Bands in inches, not based on underbust

• Uses double letters differently than the U.S.

• Bands in centimeters, based on underbust

• Typically increase in steps of 5 (80, 85, 90)

• Bands labeled 1, 2, 3, etc.

• Cups usually A–F

• Bands in centimeters, no set sizing system

• Bands start at 8 and increase by 2

• AU/NZ 8 ≈ US 30

When shopping internationally, always check the brand’s conversion chart — especially if you’re buying bras for breast forms.

Due to bra sizing varying between brands and countries the best way to find the bra size you should be wearing is working with a fitter who is familiar with the brand and fitting chart you are shopping with. It is also important to make sure that the fitter you are working with has experience working with AMAB bodies.

Looking for fitting help? Speak with our certified fitters.

Here is the most important rule of bra shopping: Forget everything you think you know about your size.

 Because there is no universal industry standard, a "40C" can look and feel wildly different depending on whose label is on the tag.

Every designer uses their own unique patterns, fit models, and fabrics, making your size a moving target rather than a fixed number. To avoid the frustration of a bad fit, treat every new brand as a fresh start:

• Trust the Chart, Not the Label: Never assume your size carries over. Always match your current measurements to that specific brand’s sizing guide.

• Verify the System: Double-check if the brand uses U.S., UK, or EU sizing, as the "double letters" and band numbers mean very different things across borders.

• Talk to a Pro: When in doubt, lean on a fitter. A professional who knows a brand's specific quirks can save you hours of trial and error.

Bra sizing doesn’t have to feel intimidating or exclusionary.

Once you understand how band and cup sizing actually work — and how international sizing differs — shopping becomes more intuitive and a lot more empowering.

Your body deserves comfort, support, and styles that help you feel confident and at home in your presentation.