I normally take a size 9 in women's shoes, sometimes 9.5. I even have (had) some 10s. If I were to select the Birkenstock European sizing that shows as equivalent, I would have selected a size 40.
I was fitted in the Birk store by a very knowledgeable salesperson. Left to my own devices, I would have bought the size 40. The saleslady showed me exactly how Birks are supposed to fit, where the arch should be, where my heel should be, where my toes should be. I bought a size 38. Then, I bought another size 38. These shoes are not a panacea for me -- but they DO enable me to walk around somewhat and carry on with normal life. Even though I don't feel cured by any stretch, I give Birkenstocks very high marks. I am going to buy more -- in a size 38 (which theoretically would translate to about an 8 in women's shoes -- and my foot is no 8.)
My point is -- if you HAVE the option of going to a Birk store and getting fitted, please take it. Otherwise, you may have a $93 pair of sandals that don't fit properly and don't help. Get a catalog, learn about the different Birk lines and how they fit differently; once you get fitted properly, ordering over the 'Net or by catalog is infinitely easier and more cost effective.