To determine your size, the number is the underbust measurement and you measure around just under your t*ts, and the letter size is the overbust measure (so you measure over your nipples at the largest part). You then can look up a table on the internet that will translate to what size that is. So you will find that sizes that are smaller around are sometimes similar, so for example a 30E is similar tit size to a 34C. I hope that makes sense. I used to work in a bra shop. I'm a 36B by the way.
from my friends I've heard that there's 2 bra sizes that 'fit' them basicly by going for a larger cup and shorter band or smaller cup and longer band. But I think you better ask who can really help you with it what bra you should wear considering its prob not healthy to wear the wrong size. Though if I were to gues id say the middle one is prob the best one for you eg 36c but I don't know really.
Usually girls tend to wear bras that have bigger straps than they need and cups that are smaller than they need. I'd say if you fit perfectly into a size 34D than you are a 34D.
If you can expand your band on the back more than two fingers worth it's too big for you (the band is too big for you). And if your cups don't embrace your entire breast properly, it cuts into it and it just doesn't feel nice at all. Measure over your breasts around your body, if the measurement is 34 than you're a 34 and a cup size up. The rule is whatever bra you're wearing now (if it's wrong, if you're wearing band size that's too big) is that you go down a band size and go up a cup size...
Hope I helped
(btw, I'm a DD/E...it's not that fun finding a bra Haha)
Some bras are diff. It depends on the brand, some brands makes their sizes smaller. See, in Taiwan I'm a D, in the west I'm a B/C, Inside Taiwan, for some brands I'm C, for others I'm a D. Sometimes I'm a 34, sometimes I'm a 36. It's normal, dw. (The girl who works there even says it, the bras in our store are smaller, so let me get you a bigger one ) The ACB thing is the size of your cups, and the number is I thinnnk the length from your back to your boobs.
How do I determine my bra size? I just go to a lingerie store and get one of the ladies to measure me. Sure, its a little awkward to get your boobies out in front of a stranger, but they are super professional about it and its much easier than speculating and guess work.
And I am a DD-E. Having the right sized bra is pretty necesssary for me.