I should have paid more attention at some point in my life, but I didn’t.
I have no idea what a “D” versus an “A” versus a “Q” even is.
I’m sure Google will take me to very interesting places, which frankly I wouldn’t mind.
But what does it all mean?
The number is the measurement around the torso. The letter is cup size, with B being larger than A, and C being larger than B, etc.
Put your bra on. Then take a tape measure and measure around the band (underneath the cups). Then measure around the fullest part of the cups. Subtract the two numbers. If the difference is 1 inch, it is an “A,” if the difference is two inches it is a “B,” and so on.
If you see a double letter, like “DD” that is the same as the next letter up. For example, “DD” is the same as “E.”
Cup size (the letter) is a measure of the difference around between the band (right under the breasts) and the fullest part of the bust. Bigger letter = bigger boobs.
For example, 36C would be the size for a woman who measures 36 inches around right under the breasts (or right above them, under the armpit), and who is about three inches bigger around measured over her breasts at the fullest part. If she’s five inches bigger around there, she’ll wear a DD (American/UK) or E (European); if she’s only two inches bigger, she’ll take a B cup. Generally, one letter = one inch bigger around; above 4", most American manufacturers use extra Ds, so 5" is DD, 6" is DDD, 7" is DDDD, etc. European cup sizes follow straight letters: 5" is E, 8" is H, etc. The UK has a weirder scheme, with doubled-up letters inserted, so 5" is DD, 6" is E, 7" is F, 8" is FF, and so. Q, under any scheme, would be a woman with a truly massive chest, well outside ‘normal’ sizing.
At the other extreme, cup size A fits a woman who is only about an inch bigger around on her bust than right below it, and AA fits women who are less than an inch bigger around. (A flat-chested woman might even wear AAA.)
Wow. What a complicated way of doing things. No wonder I hadn’t a clue.
I think your response fails at the first sentence! 
What’s the largest then?
48 OMG
p
If I recall correctly, there’s a bra company that has an alternative method of measuring bra and cup sizes. Not being a bra wearer myself (being male), I found it interesting because of my fascination with systems of measurement (including those that don’t involve breasts). The only thing I remember about it is the company using it has a purpose-designed measuring tape. It seems to me that the currently prevalent system is lacking for a few reasons, the main one being that not all breasts are the same, so good on that company for developing an alternative. (I’d do a search now for it but given the subject matter and the fact that I’m “desk warming” at work before our school vacation begins, it wouldn’t be a good idea to do such an Internet search now.)
Well, bras are intrinsically complicated garments. Think about it: you’re basically wearing a skintight chest harness that’s expected to support fairly sizable and sensitive body parts in a way that’s not only secure but comfortable, easy to put on and remove, and visually appealing.
The standard band size/cup size system, where the band size is an absolute measurement and the cup size is a relative one, is actually about the simplest possible way you could describe bra sizing. In fact, it radically over-simplifies the fitting issues, which is why different bras with the same number/letter sizing can fit very differently.
You may be thinking of Jockey’s much-publicized “Volumetric Fit” bra sizing:
Personally, if I’m going to be stuffing the girls into cups to check the fit, I’d rather try on an actual fabric bra, where I can acquire the thing immediately if I like it, than fiddle with a plastic cup from a sizing kit that may or may not accurately represent how the actual garment as a whole will fit me.
Fuck that noise.
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I’ve never heard of “Q” size. I would say she probably has back problems if she’s wearing a “Q”.
So the letter is the cup size. 
The REAL debate: The Bro vs. Manziere?
Generally normal stores don’t stock bra’s above a DD cup. Some stores will stock E cups (but then again in some stores E cup is DD). Only very specialty bra stores will stock things F cup and above (Victoria Secret stops at E cup bras oddly enough at well)
Related, average breast size tends to be all over the place depending on which study you look at but the one I trust the most came from an organization that polled American bra manufacturers directly and asked them what percentage of cup sizes they sold each, and the combined results were
AA cup - 2%
A Cup - 15%
B Cup - 44%
C Cup - 28%
D Cup - 10%
DD Cup - 0.9%
Above DD cup - 0.1%
Of course this doesn’t take into account band size as well, since a 36C could also wear a 38B or 34D bra but it’s the most accurate one I’ve seen.
So now we can move on to the next complication which is that the cup size does not actually tell you how much boob you’ve got ( volumetrically) because it’s based on a linear measurement, not a volume measurement. So a 10D bra holds breasts the same size as a 16A bra … only on a slimmer-framed woman.
The rule of thumb is, up a band size, down a cup size.
Also, having spent a small amount of time selling bras, I’m completely sure that that survey is only an accurate way of gauging the sizes of the actual customers up to about a C. Above that, the D-stats are inflated by all the large chested women buying whatever the biggest thing on the shelf is, and large sizes downgraded appropriately.
The statistics above will be wrong because, apparently, most women wear the wrong size bra anyway.
Thisis a newspaper report so should be OK for most people.
This begs the question of where does the 80% figure come from. Such studies are there are have been on small samples and often selective rather than random.
The above came from a quora question https://www.quora.com/Where-did-the-phrase-80-of-women-wear-the-wrong-bra-size-come-from
Still confusing … obviously this matter needs some hands-on exploration …