I bought my first cup after painstakingly careful research and consideration. I measured myself while on my period, and found that my vagina was 7 cm long and my cervix was 4 to 4.5 cm deep. On my heaviest day, I saturated a regular tampon in 4 to 5 hours, so I estimated that my flow rate was around 2 mL per hour. I am also small-framed, in my mid-20s, and sexually active but without children.
Combining all of these criteria, I thought that the medium soft MeLuna (ring-stem) was the most appropriate choice. Lengthwise, it matched my internal dimensions, and with its 20 mL capacity, I figured I'd only have to change it every 8 to 10 hours (my biggest desire was being able to sleep peacefully through the night). I also thought that a soft cup would be more comfortable, given the fact that I'm fairly tight down there, and I didn't want undue compression on my bladder, urethra, or rectum.
When my cup arrived and I tried it out, I immediately discovered that I probably erred by going soft, especially since I was also able to try the small klassik sample that MeLuna included with my order. Beyond my initial issues getting the cup to pop open (I did finally get it working with the labia fold and lots of fiddling -- fortunately, I am quite comfortable with my body), I discovered that the cup was so soft that it was basically being collapsed by my body, which would obviously reduce its capacity. I also found that I liked to push the cup all the way up around my cervix to ensure a good seal, which probably further diminished its capacity. I noticed that it would start to leak after 4 or 5 hours, filling only up to 10 mL.
Undeterred, I bought a large Lunette next. I figured that since it was longer, firmer, and had a much higher capacity, it would work better for my body. When it arrived, I was happy to find that not only did it pop open readily, it fit me well, did not collapse, and I could not feel it when it was in. Removal was way more difficult -- unlike my MeLuna's ring-stem, the Lunette's tab stem was much harder to grab onto and tug down on -- and I basically had to reach all the way in and push down the rim in order to break the seal.
I was fine with everything until I used it on my next cycle, and found that it, too, leaked after only five or so hours, with around 12 mL of fluid inside -- barely an improvement over the MeLuna, despite the fact that its capacity was 10 mL greater. And this time around, I can't blame it on a soft cup that was being collapsed. Admittedly, I do lose capacity to my cervix since I push the cup as far up as it can go -- otherwise, the stem pokes me, but I cannot trim it because the cup works itself up naturally and I need it for removal -- but I was honestly still expecting a bigger difference in effective capacity, since I also wore the MeLuna high.
To study the capacities of my two cups, I then did an experiment. I took both cups, and filled them with water. I tilted them at a 45 degree angle to simulate its position inside a vagina, pressed my fingertips together to form the shape of a cervix, and dipped them 2.5 cm inside. When I measured the water that remained, I got 10 to 12 mL -- which was exactly what I observed to be the cups' maximum capacities when I was using them.
Now, up until that point, I was still willing to entertain the notion that perhaps I was still on the learning curve, and leaking due to an ill-fitting cup or a newbie mistake. But now, I'm beginning to think that there is a ceiling to a cup's leak-free capacity, and it is nowhere near the 20 or 30 mL that is advertised. I must say that I am a bit disappointed, because not only do I think I have a fairly typical vagina, but my periods aren't even all that heavy. But right now, on my heavy days, I have to empty my cup at the same frequency as I have to change my tampon, which does not make the cup more convenient than disposables.
With that said, I still do like the cup because I don't have to worry about carrying disposable products, and on my lighter days, I don't have to worry about TSS or drying out my vagina, and in the long run, a cup still wins out economically and environmentally. But I do wonder whether any veteran cup users can comment on whether or not their actual, true, realistic cup capacity tops out at 10 or 12 mL like mine does, and if not, whether they can offer any suggestions for improving capacity -- whether it's an even larger cup (are there any?!), wearing them lower, or anything else. Any thoughts would be much appreciated!