There's some good advice on www.brabandproject.com - it has a calculator in the FAQ section.
The only good way of finding a fit that flatters is to try, try and try. Always go for a tight underband. Unfortunately, even when I've found my perfect size and fit in one brand, I'm completely different in another.
And don't get me started on products that seem to be popular being messed with or discontinued!
I know i'm not wearing the right size, simply because i can't find them in shops nor afford specially made. I generally squeeze into a 38C (but it digs in and leaves a big red indented ring right round my body, sometimes cutting in til i bleed) or one shop i can get 40B unwired, which fits much better round the body but they lack support and make me look flat chested. My other issue is my right breast is a comfortable C cup size in a 38 back, but my left breast won't fill the cup out and has gaping, it is a small B at largest. Finding a 40B took years, as for 40A, they seem simply to not exist, any back size over a 36 you can't get A cups, there's this silly assumption all bigger women have bigger breasts, well i certainly don't have much breast tissue and i don't gain fat there either.
Have you thought about trying a non sized bra like a bralet, crop top style or pull on these things aren't sized by cup and band rather clothes size and are normally really stretchy with no diggy wires or hooks so you can step in to them and pull them on they will fit most boob sizes you can get them with/without padding some have moulded cups, i find them good for me because my size doesn't exist in a lot of shops
Ruby I think you're right about that, a friend of mine has quite small breasts that I would have guessed to be a B/C at most, and she was professionally measured as a DD! I thought DDs were supposed to be massive, I guess it depends on the back size too though as a smaller back size generally = larger cup size. If they really have turned to vanity sizing then that would explain why actual large bras are so hard to come by, as most shops still only sell B-DD which aren't even that big now, unless you want to wear a flimsy plain white bra. I swear when I was a teenager DD bras were still huge?!
Yeah, I think we're quite close in age and I remember when I was in my early teens DD bras were massive, but now everyone and their granny says they're a G cup. I have a female relative who has boobs the size of Jordan's - they're natural and seriously huge - and I wonder how people her size can find a bra to fit. By today's sizes she's probably a PPP! I don't even think the vanity sizing achieves anything - the label on the bra doesn't change the fact that my boobs are really small!
I would not mind learning what cup size I actually am... I tried getting fitted three times, and I ended up with three different results. My problem is that my boobs are slightly unusual shapewise; I have a broad ribcage and my boobs are kind of "wide" rather than "classically big". That often means that with smaller cups I get these unsightly sideboobs, and bigger cups are a way too "deep" for me... How magnificent.
Hi Briona I understand where you are coming from. I'm a 38 F/FF but my boobs are teardrop shape. Very little flesh at the top of my bust, but plenty towards the nipples. I find that a lace bra with a small mount of stretch, but with thicker side panels (plunge style) no padding, works best for me. I can't wear a balcony bra to save my life. Have a look for a lace plunge style, it should keep them nestled together rather than trying to escape under your arms, if you know what I mean. Lol.
Thanks for the tips, Louise! 
Hi Briona I understand where you are coming from. I'm a 38 F/FF but my boobs are teardrop shape. Very little flesh at the top of my bust, but plenty towards the nipples. I find that a lace bra with a small mount of stretch, but with thicker side panels (plunge style) no padding, works best for me. I can't wear a balcony bra to save my life. Have a look for a lace plunge style, it should keep them nestled together rather than trying to escape under your arms, if you know what I mean. Lol.
I'm a 30B/C (depending on the brand) but I seem to be the same shape as you. It's so hard getting bras to fit as they usuall gape at the top as I don't have the breast tissue or body fat (as I'm v. slim) to fill it all the way up but if I go down a cup size I spill out over the top. Balconette bras are the enemy with me!
Hi Briona I understand where you are coming from. I'm a 38 F/FF but my boobs are teardrop shape. Very little flesh at the top of my bust, but plenty towards the nipples. I find that a lace bra with a small mount of stretch, but with thicker side panels (plunge style) no padding, works best for me. I can't wear a balcony bra to save my life. Have a look for a lace plunge style, it should keep them nestled together rather than trying to escape under your arms, if you know what I mean. Lol.
I'm a 30B/C (depending on the brand) but I seem to be the same shape as you. It's so hard getting bras to fit as they usuall gape at the top as I don't have the breast tissue or body fat (as I'm v. slim) to fill it all the way up but if I go down a cup size I spill out over the top. Balconette bras are the enemy with me!
I get this too! It's soo annoying! And I'm not slim at all I am a size 16 on top x
Almost everyone wears a the wrong back size (too big) and therefore cup size too small to compensate. I wish they wouldn't because then it might be easier for me to find shops that stock my size.
And I really wish Lovehoney stocked lingerie with proper back and cup sizes
100000%% agree with you, im a 30 back which are impossible to get hold of ... I wonder how many people should be in a 30 or 28 even but are wearing 32 as thats whats the shops stock x
Me too! I've begged on the pander to me thread for proper bra sizes (I hate this S/M/L rubbish) with small backs (I'm a 30' too and when I've tried size small Lovehoney bras I think the backs are at least a 34) and styles with underwiring and padding.
I love Lovehoney bodystockings, chemises, pants and stockings but the bras just do not flatter small boobs (IMHO). I need a small back side and some decent padding and support to scrape together and make the most of what little I have!
I went into an Ann Summers store once, wearing a 40D bra that was 'comfortable' and I'd been wearing for years (size not just that bra). The lady offered to measure me and I nearly passed out when she said I was in fact a 38E or I could get away with a 36G from some shops. I knew I was big busted but not THAT big!
Ok, so we've been meaning to measure my wife for a while, and after seeing this thread we finally got round to it today...
Underbust measurement is: 30-31"
In a lightly padded bra the full bust measurement is 38"
Nude or in a lace bra I measured 36" (she was very impressed that her nude measurement and bra-ed measurement were the same! - lol)
So difference of 6" ish...
So, according to the above I believe that makes her approximately a 30E (or 32DD) ?!?
Just posting really to confirm that we've got it right above - She normally wears a 34/36 C, and although the cups don't always fit as well as they could, the backstrap isn't flapping around loosely like I'd think it would based on the above.
Where does one go for a 'professional fitting'? I probably won't be able to go anywhere where I live, but it would still help me knowing where would you go in the UK or US? Briona, where did you do your fitting(s)? In one of my next visits in one of these countries, I might just do that!
I think I nailed it about a couple of years ago finally, after decades of suffering with bras, but its only by self-measuring and putting to test in shops my new size. I'm a 42DD, and I can get away with a 40E, but not a 44D...
Ok, so we've been meaning to measure my wife for a while, and after seeing this thread we finally got round to it today...
Underbust measurement is: 30-31"
In a lightly padded bra the full bust measurement is 38"
Nude or in a lace bra I measured 36" (she was very impressed that her nude measurement and bra-ed measurement were the same! - lol)
So difference of 6" ish...
So, according to the above I believe that makes her approximately a 30E (or 32DD) ?!?
Just posting really to confirm that we've got it right above - She normally wears a 34/36 C, and although the cups don't always fit as well as they could, the backstrap isn't flapping around loosely like I'd think it would based on the above.
Hence the confusion and wanting to check - lol
Thanks my fellow Honies!
i think you measured the underbust wrong you add 4 or 5 if it's an odd number to the ones you got making 34-36 the right band size
check your measurements against http://www.calculator.net/bra-size-calculator.html it's the most accurate site i've used
Ok, so we've been meaning to measure my wife for a while, and after seeing this thread we finally got round to it today...
Underbust measurement is: 30-31"
In a lightly padded bra the full bust measurement is 38"
Nude or in a lace bra I measured 36" (she was very impressed that her nude measurement and bra-ed measurement were the same! - lol)
So difference of 6" ish...
So, according to the above I believe that makes her approximately a 30E (or 32DD) ?!?
Just posting really to confirm that we've got it right above - She normally wears a 34/36 C, and although the cups don't always fit as well as they could, the backstrap isn't flapping around loosely like I'd think it would based on the above.
Hence the confusion and wanting to check - lol
Thanks my fellow Honies!
i think you measured the underbust wrong you add 4 or 5 if it's an odd number to the ones you got making 34-36 the right band size
check your measurements against http://www.calculator.net/bra-size-calculator.html it's the most accurate site i've used
Hahaha - you are correct - many thanks :)
We were using this guide: http://direct.asda.com/george/grg_womens_braSizeguide,default,pg.html
However I missed the section that added 4" to the underbust to give you the band :P
30 underbust = 34band + 36 bust gives 34C, which means she is a 34C :)
Where does one go for a 'professional fitting'? ...
I went to a specialist lingerie shop in the nearest (small!) town (Chateaubriant in France) and the lady owner pulled out a tape measure. Her eye did the rest and she gave me a few bras to try on from which I only had to find the most comfortable fit for me. She also gave me a piece of advice that I'll pass on here: When you have fastened your bra, take your breast with the opposite hand and settle it more comfortably in the cup - that does make a difference!
Of course, the service was reflected in the price, but wth, it was a once in a lifetime expense as I now have a starting point, size-wise.
I'm pretty sure there's lingerie shops in most countries, it's just a question of finding one with knowledgeable staff. I guess I was lucky...
Or you could go to any clothing store or possibly supermarket (ASDA) that sells bras, grab as many as you are allowed to in different sizes and try them on. Finding a bra that fits isn't all that different from finding a pair of shoes that fit!
Though, actually, now that I think of it, the quickest way would be: Measure under your bust (and remember the number). Take the tape measure to the store with you and measure the bra's breastbands. Take only those bras whose breastband matches your measurement and then try the different cup sizes.
Because the breastband must fit, if it's too small the bra is uncomfortable, if it's too large it will probably look bad (haven't experienced that case, though).
Really, life would be so much easier if the bra size were equal to the length of the breast band!
Yeah, I admit I was also in the past wearing wrong bra. My size now, that I lost some weight, went down one size round the bust, but my issue is that the breast seems to go up. So from 38E I am down to 36F or G, depending on the manufacturer and bra type 
Its nearly impossible to get a bra with this cup size...
Actually, I think I envy you. I'm pretty sure that when I manage to lose my extra kilos my breasts will shrink to next ot nothing - and even now there's one chain store with a wonderful selection of not very expensive bras which doesn't stock 100B (my perfect size) at all - it's from 100C upwards. Whereas a quick google search for 36F bras results in hundreds of bras...
(Okay, I'll stop complaining, there's also a good selection of 100A bras...)
Thanks Talia, but in that case I think I'm good already. I have done the measuring myself (with help from my husband) and have done the 'going to a shop / trying different sizes on' part and I am now quite happy with the end result of it all. I just thought a "professional fitting" implied more than this DIY system, so I thought I was missing out. Apparently not though, so I'm all good. I think I have the right size anyway (finally, after decades of suffering), was just curious to check.
I think as with all clothes there are no real shortcuts. It's just a question of getting the basic measurements right (I'm sorry to say that there's online shops that have more useful measurement/size tables than LH) and then for the details it's a matter of working your way through the different manufacturers and styles and finding out what fits and what doesn't.
(Of course, then you need to factor in looks, and price, and you better don't have any allergies to any of the materials, and... I understand why my mother would often moan "Why can't I live in a place where I could run around naked?")