The short answer is that the smallest standard cup size is AA, the smallest band most retailers stock is 28, and the size most often described as the smallest overall is a 28AA.
That is the headline. But because a bra size is always a band and a cup together, "smallest" is a slightly bigger question than it first looks, and the full answer is more useful than the headline alone.
This guide gives you the plain facts, explains the part that trips most people up, and, if you have a smaller bust yourself, shows you that your size really does exist and is easy to find.
In short
The quick answer
If you only need the facts, here they are.
- The smallest standard cup is AAIt is one step smaller than an A cup.
- AAA cups exist but are rareOnly a few specialist brands make them.
- The smallest common band is 28A 26 band turns up occasionally from specialist makers.
- 28AA is the size most often called the smallestA 28-inch band with a very shallow cup.
- There is no single smallest sizeBand and cup always work together, so it depends which you mean.
What is the smallest bra size?
Taken in full, the answer has two parts, because every bra size has two parts.
The smallest standard cup size is AA. A handful of specialist brands make an AAA cup, which is smaller still, but it is rare and not something most shops stock. The smallest band size that is widely available is 28, with a 26 band turning up now and then from specialist makers.
Put the two together and the size usually named as the smallest is a 28AA: a 28-inch ribcage measurement, with a bust less than an inch larger than that. It is a real, manufactured, stocked size, not a special order.
Why there is no single smallest size
Here is the part that the quick answer cannot quite capture. A bra size is never just a letter or just a number. It is always the two together, and that changes what "smallest" means.
The smallest cup letter is one answer. The smallest band is another. Someone right at the bottom of the scale might wear a 28AA, or more rarely a 28AAA. So there is no single tidy "smallest bra size", and any page that gives you just one number is leaving out the bit that makes the answer useful.
Once you see a size as a band and a cup together, the question stops being "what is the smallest" and becomes "what is the smallest that fits me" - and that one has a real answer.
The rest of this guide is about getting you to that answer.
Why AA is smaller than A
This one catches almost everyone out, so it is worth saying plainly. With most cup sizes, a double letter is bigger. A DD is bigger than a D, an FF is bigger than an F, and so on up the scale.
The A sizes are the exception, and they run the other way. An A is the largest of the three, an AA is smaller, and an AAA is the smallest of all. So at this end of the scale, more A's means a smaller cup, not a bigger one.
It is truly counterintuitive, and there is no neat reason for it - it is simply how the sizing grew up. Worth knowing before you read any size chart, because it is the single most common point of confusion at the small end.
Cup size depends on band size
The other thing worth understanding is that a cup letter is not a fixed amount. It is a difference measurement - the gap between your ribcage and the fullest part of your bust. An AA cup means a difference of around half an inch, an A cup around one inch.
Because it is a difference, the same letter holds more on a bigger band. A 28AA, a 32AA and a 36AA all share a cup letter, but the actual cup gets roomier as the band grows.
The same AA cup, three band sizes28AAsmallest cup volume32AAmore room, same letter36AAroomier stillA bigger band carries a bigger cup, even when the letter does not change.
An AA cup is not a fixed amount. The same letter holds more as the band size goes up.
Try it for yourself below, then read on for how to work out your own size.
See how a cup letter changes
Pick a band size and see what an AA cup means on it.
Choose a band size:
Pick a band size above to see what an AA cup means on it.
How to work out your own size
Working out where you sit on the scale comes down to two measurements.
- Your band size is the measurement around your ribcage, directly under the bust, in inches. That number is your band.
- Your cup size is the difference between your ribcage measurement and the fullest part of your bust. Around one inch of difference is an A, and less than that is an AA.
That is the principle in brief. For the full step-by-step, our bra size calculator works it out for you in a moment, and the fitting guide walks through measuring properly. If you would like the wider picture of how the sizing system fits together, our guides to how bra sizes work and bra sizes explained cover it in more depth.
Is my size too small to find?
If you have a smaller bust and the high street has let you down, it is easy to conclude that your size simply is not made. It almost certainly is.
What you have run into is a stocking gap, not a manufacturing one. Large chains tend to carry a narrow band of the most popular sizes, so the smaller end gets thin coverage on the shelf. That is a choice about what sells in volume, not a sign that AA cups or 28 bands do not exist. They are standard, manufactured sizes.
Specialist retailers that carry a full size range stock the small end properly. At Uplifted Lingerie we stock from a 28AA upward, across named brands, so a smaller bust is properly catered for here rather than treated as an afterthought. Whatever your size, there is a good chance it is on the shelf.
Styles that suit a smaller bust
With a smaller bust, the choice of style is about shape and comfort rather than heavy support. Here are four cuts that tend to work well.
Shape 
Balcony bras
An open, lifted cut that shapes from below and gives a defined silhouette under clothes.
Best for: a defined shape and a little lift.
Shop balcony bras
Low front 
Plunge bras
Cut low and close at the centre, so it sits flush against the breast and works well under open necklines.
Best for: lower necklines and a close, flush fit.
Shop plunge bras
Smooth 
Padded bras
A lightly padded cup adds a little smooth shape and a clean line, and keeps its form against the body.
Best for: a smooth shape and a defined cup line.
Shop padded bras
Everyday 
T-shirt bras
A smooth, seam-free everyday cup that sits cleanly against the breast and works under almost anything.
Best for: a reliable, invisible daily bra.
Shop t-shirt bras
We carry bras from a 28 to a 48 back and in cups from AA well past HH, so wherever you sit on the scale, there is a good chance your size is here. And if a bra arrives and the fit is not quite right, our no-quibble returns policy means swapping it for another size costs you nothing but a moment.
Still not sure? Ask a person
If you would like a second opinion on your size, you do not have to work it out alone. Our fitting team is happy to help by phone or email, the kind of advice you would once have had over a shop counter. You can get in touch here, or work through the fitting guide and size calculator at your own pace first.
Common questions about the smallest bra size
What is the smallest bra size?
The smallest standard cup size is AA, and the smallest band size widely stocked is 28, so the size most often described as the smallest overall is a 28AA. A few specialist brands make an AAA cup, which is smaller again, but it is rare. Because a bra size is always a band and a cup together, there is no single tidy answer, but 28AA is the size usually meant.
Is AA smaller than A?
Yes. At the small end of the scale the letters run the opposite way to the rest of the alphabet, so an A cup is the largest of the three, an AA is smaller, and an AAA is the smallest. It is the reverse of the DD and FF pattern further up the scale, where a double letter means a bigger cup.
Is 32A a small bra size?
A 32A sits towards the smaller end of average, but it is not the smallest. The 32 is a fairly common band size, and an A cup is one step up from AA. Remember that cup volume is relative to the band, so a 32A holds a little more than a 28A even though they share a cup letter.
What is the second smallest bra size?
If you are thinking of cup letters, AAA is the smallest and AA is the second smallest, with A next. If you are thinking of band sizes, a 26 band is the smallest you will occasionally find and 28 is the next size up. As ever, it depends on whether you mean the cup or the band.
What does 28AA mean?
The 28 is the band size, which is the measurement around the ribcage in inches, so a 28 band fits a 28-inch ribcage. The AA is the cup size, which is the difference between the ribcage and the fullest part of the bust. An AA means that difference is around half an inch, so a 28AA is a small band with a very shallow cup.
Can you get a bra smaller than AA?
Yes, in the cup. A few specialist brands make an AAA cup, which is smaller than an AA. It is not widely stocked, so it can take some searching. In the band, a 26 is the smallest size that turns up, and again it is rare. For most people, a 28AA is the smallest size they will need and it is far easier to find.