Bra Sizes Explained: What the Number and the Letter Actually Mean

Quick answer: your bra size is two things stitched together.

The number represents your band size, based on the measurement around your ribcage directly under your bust. In Australian sizing, it is a size label rather than the measurement itself, so an AU 12 corresponds to a particular underbust range.

The letter is the difference between that ribcage measurement and the fullest part of your bust. It's not a fixed volume. That is why an E cup on a size 10 band has a different cup volume from an E cup on a size 18 band, even though the letter is the same. 

The rest of this page unpacks that, with real Aussie sizes people actually search.

The number: your band size

In Australia, band sizes run 8, 10, 12, 14, 16, 18 and up. That number is the measurement around your ribcage, right under your bust. It's not a dress size. It's not a personality type. It is calculated from your underbust measurement, using the brand’s sizing chart. 

The band is also doing most of the work. Around 80 to 90% of a bra's support comes from the band, not the straps and not the cups. In wire-free bras that number matters even more, because there's no wire holding the shape for you. Our Fit whisperers see it every day. A band that rides up at the back is often too loose. Digging at the front can also point to an issue with the cup size, wire shape or overall fit, so it is worth checking the whole bra rather than adjusting the band alone.

The letter: your cup size (and why it's not a fixed size)

Here's the bit almost nobody explains properly. The cup letter isn't a size in the way a shoe is a size. It's a difference. Specifically, the difference between your bust measurement and your band measurement.

Your cup size is determined by comparing your underbust and fullest-bust measurements against the brand’s size chart. As the difference between those measurements increases, the cup size generally increases too. Because sizing conventions and letter sequences vary, the chart is more reliable than assigning a letter from a simple centimetre calculation. 

In Nala’s Australian sizing, the cup letters run A, B, C, D, DD, E, F, G, H, J and K. Other sizing systems use different sequences, which is why the country or system matters once you move beyond a D cup. Yes, DD is a real step between D and E. It's not "double D drama" and it's not a made-up size. It's one 2.5 cm step above D, and one step below E. 

Why the same letter fits differently on different bands

Because the cup letter is a difference, the actual volume of the cup depends on the band it sits on. A D cup on a size 10 band is a much smaller cup than a D cup on a size 18 band. Same letter. Very different amount of boob.

This is why two friends can both call themselves "a D cup" and hold wildly different volumes, and why the letter alone tells you almost nothing without the number in front of it.

It's also the reason bra sizes have a bit of stretch in them. If your band goes up a size, your cup letter usually goes down a step to hold the same volume, and vice versa. That trick has a name: sister sizing. Handy to know when your usual size is out of stock or almost right but not quite.

What 12E, 34DD, 32F and 14C actually mean

Let's put real sizes through it.

Notice what happens when you look across the rows. 34DD and 12E both sit on roughly the same ribcage, and the cup difference is only one step apart. They're much closer neighbours than the letters suggest. And "is 34DD big?" is honestly the wrong question. On a small band it's a moderate cup. On a bigger band the same letter would be a bigger cup. The letter on its own does not tell you a volume.

Reading a size off an overseas bra

If the size on the label looks unfamiliar, it's probably from a different system. Australian and UK band numbers are different, but the maths lines up. AU 12 is roughly UK 34, AU 14 is UK 36, and so on. US sizing uses inches for the band too, and the cup letters start to diverge above D (US often skips DD or uses different lettering higher up). If you want to translate a size properly, our international sizing conversions guide has the full chart.

What is a 'normal' bra size?

There's no such thing as a normal bra size, and anyone giving you a single number is guessing. Reported Australian averages tend to sit somewhere around a 12C to 14C, but "average" is a statistic, not a target. Real bodies spread across the whole range.

Cup letters go well past G in Australian sizing. The biggest cup a brand offers depends on the brand, not on some universal ceiling. The letter is not a scorecard. It's not a personality. It's the gap between two tape measurements, and that's genuinely all it is.

Got your size sorted?

If you already know your size and now you know what it means, you can go shop it in the Nala bra collection, including plenty of wire-free options across the range.

If you've landed here without a size yet, the next step is measuring. We have a full walk-through on how to measure your bra size, tape and all, so you can come back to this page and know exactly what your number and letter are telling you.

Frequently asked questions

What does DD mean on a bra?

DD is a real cup size that sits one step above D and one step below E in Australian and UK sizing. Each letter step is roughly 2.5 cm of difference between your bust and your band, so DD is about 2.5 cm more cup volume than D on the same band. It's not "extra large D", it's just the next step up.

Is a 34DD big?

Not really, and not by itself. 34DD sits on a 34-inch band (roughly an AU 12) with a DD cup, which is one step above D. On a small band that's a moderate cup. The letter alone never tells you a volume without the band number in front of it, so DD on a 34 is very different to DD on a 44.

 

What is a D cup?

A D cup means the difference between your bust and underbust measurements falls within the D-cup range for that brand and sizing system. It is not a fixed volume, so a 10D and an 18D have different cup capacities. 

What is the difference between a cup A and a cup B?

About 2.5 cm of difference between your bust and your band. A is a smaller gap, B is one letter step bigger. Every letter step in Australian and UK sizing works the same way: one letter equals roughly 2.5 cm more or less of cup, sitting on the same band.

 

What is the largest cup size for a bra?

There's no universal maximum. Australian cup letters keep going past G, GG, H, HH and beyond, and how far a brand goes depends on the brand. If you sit at the larger end of the range, our bras for bigger busts collection is the place to start.

 

Why is my cup letter different in different brands?

Because brands cut cups slightly differently, use different fabrics and finish bands with different amounts of stretch. The maths behind the letter is the same, but a C in one label can fit like a D in another. It's normal, and it's why trying your usual size and one sister size is often the fastest way to land on a fit.